Allow themes to define output formats, media types and params

This allows a `config.toml` (or `yaml`, ´yml`, or `json`)  in the theme to set:

1) `params` (but cannot override params in project. Will also get its own "namespace", i.e. `{{ .Site.Params.mytheme.my_param }}` will be the same as `{{ .Site.Params.my_param }}` providing that the main project does not define a param with that key.
2) `menu` -- but cannot redefine/add menus in the project. Must create its own menus with its own identifiers.
3) `languages` -- only `params` and `menu`. Same rules as above.
4) **new** `outputFormats`
5) **new** `mediaTypes`

This should help with the "theme portability" issue and people having to copy and paste lots of setting into their projects.

Fixes #4490

helpers/path.go

@@ -154,11 +154,16 @@ func ReplaceExtension(path string, newExt string) string {
 // AbsPathify creates an absolute path if given a relative path. If already
 // absolute, the path is just cleaned.
 func (p *PathSpec) AbsPathify(inPath string) string {
+	return AbsPathify(p.workingDir, inPath)
+}
+
+// AbsPathify creates an absolute path if given a working dir and arelative path.
+// If already absolute, the path is just cleaned.
+func AbsPathify(workingDir, inPath string) string {
 	if filepath.IsAbs(inPath) {
 		return filepath.Clean(inPath)
 	}
-
-	return filepath.Join(p.workingDir, inPath)
+	return filepath.Join(workingDir, inPath)
 }
 
 // GetLayoutDirPath returns the absolute path to the layout file dir