✨ Implement Page bundling and image handling

This commit is not the smallest in Hugo's history.

Some hightlights include:

* Page bundles (for complete articles, keeping images and content together etc.).
* Bundled images can be processed in as many versions/sizes as you need with the three methods `Resize`, `Fill` and `Fit`.
* Processed images are cached inside `resources/_gen/images` (default) in your project.
* Symbolic links (both files and dirs) are now allowed anywhere inside /content
* A new table based build summary
* The "Total in nn ms" now reports the total including the handling of the files inside /static. So if it now reports more than you're used to, it is just **more real** and probably faster than before (see below).

A site building  benchmark run compared to `v0.31.1` shows that this should be slightly faster and use less memory:

```bash
▶ ./benchSite.sh "TOML,num_langs=.*,num_root_sections=5,num_pages=(500|1000),tags_per_page=5,shortcodes,render"

benchmark                                                                                                         old ns/op     new ns/op     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      101785785     78067944      -23.30%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     185481057     149159919     -19.58%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      103149918     85679409      -16.94%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     203515478     169208775     -16.86%

benchmark                                                                                                         old allocs     new allocs     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      532464         391539         -26.47%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     1056549        772702         -26.87%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      555974         406630         -26.86%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     1086545        789922         -27.30%

benchmark                                                                                                         old bytes     new bytes     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      53243246      43598155      -18.12%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     105811617     86087116      -18.64%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      54558852      44545097      -18.35%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     106903858     86978413      -18.64%
```

Fixes gohugoio#3651
Closes gohugoio#3158
Fixes gohugoio#1014
Closes gohugoio#2021
Fixes gohugoio#1240
Updates gohugoio#3757

Gopkg.toml

@@ -21,8 +21,8 @@
  revision = "v1.1.0"
 
 [[constraint]]
-  branch = "master"
-  name = "github.com/dchest/cssmin"
+ name = "github.com/disintegration/imaging"
+ revision = "v1.2.4"
 
 [[constraint]]
   name = "github.com/magefile/mage"
@@ -116,6 +116,10 @@
   name = "github.com/stretchr/testify"
   version = "1.1.4"
 
+[[constraint]]
+  branch = "master"
+  name = "github.com/olekukonko/tablewriter"
+
 [[constraint]]
   name = "github.com/yosssi/ace"
   version = "0.0.5"

commands/benchmark.go

@@ -48,12 +48,7 @@ func init() {
 }
 
 func benchmark(cmd *cobra.Command, args []string) error {
-	cfg, err := InitializeConfig(benchmarkCmd)
-	if err != nil {
-		return err
-	}
-
-	c, err := newCommandeer(cfg)
+	c, err := InitializeConfig(false, nil, benchmarkCmd)
 	if err != nil {
 		return err
 	}
@@ -84,7 +79,7 @@ func benchmark(cmd *cobra.Command, args []string) error {
 
 	t := time.Now()
 	for i := 0; i < benchmarkTimes; i++ {
-		if err = c.resetAndBuildSites(false); err != nil {
+		if err = c.resetAndBuildSites(); err != nil {
 			return err
 		}
 	}

commands/commandeer.go

@@ -18,14 +18,18 @@ import (
 	"github.com/gohugoio/hugo/deps"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugofs"
+	src "github.com/gohugoio/hugo/source"
 )
 
 type commandeer struct {
 	*deps.DepsCfg
 	pathSpec    *helpers.PathSpec
 	visitedURLs *types.EvictingStringQueue
 
+	staticDirsConfig []*src.Dirs
+
 	serverPorts []int
+	languages   helpers.Languages
 
 	configured bool
 }
@@ -44,29 +48,33 @@ func (c *commandeer) PathSpec() *helpers.PathSpec {
 	return c.pathSpec
 }
 
-func (c *commandeer) languages() helpers.Languages {
-	return c.Cfg.Get("languagesSorted").(helpers.Languages)
-}
-
 func (c *commandeer) initFs(fs *hugofs.Fs) error {
 	c.DepsCfg.Fs = fs
 	ps, err := helpers.NewPathSpec(fs, c.Cfg)
 	if err != nil {
 		return err
 	}
 	c.pathSpec = ps
+
+	dirsConfig, err := c.createStaticDirsConfig()
+	if err != nil {
+		return err
+	}
+	c.staticDirsConfig = dirsConfig
+
 	return nil
 }
 
-func newCommandeer(cfg *deps.DepsCfg) (*commandeer, error) {
-	l := cfg.Language
-	if l == nil {
-		l = helpers.NewDefaultLanguage(cfg.Cfg)
-	}
-	ps, err := helpers.NewPathSpec(cfg.Fs, l)
-	if err != nil {
-		return nil, err
+func newCommandeer(cfg *deps.DepsCfg, running bool) (*commandeer, error) {
+	cfg.Running = running
+
+	var languages helpers.Languages
+
+	if l, ok := cfg.Cfg.Get("languagesSorted").(helpers.Languages); ok {
+		languages = l
 	}
 
-	return &commandeer{DepsCfg: cfg, pathSpec: ps, visitedURLs: types.NewEvictingStringQueue(10)}, nil
+	c := &commandeer{DepsCfg: cfg, languages: languages, visitedURLs: types.NewEvictingStringQueue(10)}
+
+	return c, nil
 }