tpl: Add humanize func and cleanup lint

Add humanize (inflect.Humanize) to the template funcMap.  Documentation and
tests are included.

Various code cleanups of the template funcs:

- Break pluralize and singularize out into stand-alone funcs.
- Sort the list of funcMap entries.
- Add some minimal godoc comments to all public funcs.
- Fix some issues found by golint and grind.

content/templates/functions.md

@@ -387,6 +387,16 @@ Takes a string of code and a language, uses Pygments to return the syntax highli
 Used in the [highlight shortcode](/extras/highlighting/).
 
 
+### humanize
+Humanize returns the humanized version of a string with the first letter capitalized.
+
+e.g.
+```
+{{humanize "my-first-post"}} → "My first post"
+{{humanize "myCamelPost"}} → "My camel post"
+```
+
+
 ### lower
 Converts all characters in string to lowercase.