add --section-path, rename --tag to --section-tag

this flag is for loading a .lit file in the context of another .lit
file, e.g. loading docs and rendering release notes that reference the
docs

Signed-off-by: Alex Suraci <suraci.alex@gmail.com>

booklitcmd/command.go

@@ -19,10 +19,11 @@ import (
 type Command struct {
 	Version func() `short:"v" long:"version" description:"Print the version of Boooklit and exit."`
 
-	In string `long:"in"  short:"i" required:"true" description:"Input .lit file to load."`
-
+	In  string `long:"in"  short:"i" required:"true" description:"Input .lit file to load."`
 	Out string `long:"out" short:"o" description:"Directory into which sections will be rendered."`
-	Tag string `long:"tag" short:"t" description:"Section tag to render."`
+
+	SectionTag  string `long:"section-tag"  description:"Section tag to render."`
+	SectionPath string `long:"section-path" description:"Section path to load and render with --in as its parent."`
 
 	SaveSearchIndex bool `long:"save-search-index" description:"Save a search index JSON file in the destination."`
 
@@ -116,13 +117,18 @@ func (cmd *Command) Build() error {
 	}
 
 	sectionToRender := section
-	if cmd.Tag != "" {
-		tags := section.FindTag(cmd.Tag)
+	if cmd.SectionTag != "" {
+		tags := section.FindTag(cmd.SectionTag)
 		if len(tags) == 0 {
-			return fmt.Errorf("unknown tag: %s", cmd.Tag)
+			return fmt.Errorf("unknown tag: %s", cmd.SectionTag)
 		}
 
 		sectionToRender = tags[0].Section
+	} else if cmd.SectionPath != "" {
+		sectionToRender, err = processor.LoadFileIn(section, cmd.SectionPath, basePluginFactories)
+		if err != nil {
+			return err
+		}
 	}
 
 	if cmd.Out == "" {

load/processor.go

@@ -25,7 +25,11 @@ type parsedNode struct {
 }
 
 func (processor *Processor) LoadFile(path string, pluginFactories []booklit.PluginFactory) (*booklit.Section, error) {
-	section, err := processor.EvaluateFile(nil, path, pluginFactories)
+	return processor.LoadFileIn(nil, path, pluginFactories)
+}
+
+func (processor *Processor) LoadFileIn(parent *booklit.Section, path string, pluginFactories []booklit.PluginFactory) (*booklit.Section, error) {
+	section, err := processor.EvaluateFile(parent, path, pluginFactories)
 	if err != nil {
 		return nil, err
 	}

section.go

@@ -112,7 +112,11 @@ func (con *Section) Number() string {
 
 	parentNumber := con.Parent.Number()
 	selfIndex := 1
-	for i := 0; con.Parent.Children[i] != con; i++ {
+	for _, child := range con.Parent.Children {
+		if child == con {
+			break
+		}
+
 		selfIndex++
 	}