Improve separation between templates and static content

- Move static content into "static" dir instead of "www".
- Stop using `router.NotFound` in favor of `router.ServeFiles` with a
  named prefix. This constrains the access to assets that start with
  "/static/", hence accomplishing one of the goals.

Relates to #2

README.md

@@ -15,7 +15,7 @@ Here's what I was trying to accomplish:
 1. Use some templates as partials (`header.hmtl`, `footer.html`).
 1. Serve static content in a manner similar to
    [http.FileServer](https://golang.org/pkg/net/http/#example_FileServer).
-1. ~~Exclude templates from the static files being served.~~
+1. Exclude templates from the static files being served.
 1. Support custom template functions.
 1. Compile everything into a single static binary (including templates
    and static files).
@@ -36,7 +36,7 @@ cd $GOPATH/src/github.com/jmcfarlane/golang-templates-example
 go generate
 go build && ./golang-templates-example
 curl http://localhost:8080/hello/jack
-xdg-open http://localhost:8080/golang.png
+xdg-open http://localhost:8080/static/golang.png
 ```
 
 **NOTE:**

main.go

@@ -1,6 +1,6 @@
 package main
 
-//go:generate go-bindata-assetfs www/... templates/...
+//go:generate go-bindata-assetfs static/... templates/...
 
 import (
 	"html/template"
@@ -52,8 +52,8 @@ func main() {
 	// Example route that takes one rest style option
 	router.GET("/hello/:name", hello)
 
-	// Serve static assets via the "www" directory
-	router.NotFound = http.FileServer(assetFS())
+	// Serve static assets via the "static" directory
+	router.ServeFiles("/static/*filepath", assetFS())
 
 	// Serve this program forever
 	log.Fatal(http.ListenAndServe(":8080", router))