Use markdown frontmatter to provide Table of contents, language and frontmatter rendering (#11047)

* Add control for the rendering of the frontmatter
* Add control to include a TOC
* Add control to set language - allows control of ToC header and CJK glyph choice.

Signed-off-by: Andrew Thornton art27@cantab.net

go.mod

@@ -124,6 +124,7 @@ require (
 	gopkg.in/ini.v1 v1.52.0
 	gopkg.in/ldap.v3 v3.0.2
 	gopkg.in/testfixtures.v2 v2.5.0
+	gopkg.in/yaml.v2 v2.2.8
 	mvdan.cc/xurls/v2 v2.1.0
 	strk.kbt.io/projects/go/libravatar v0.0.0-20191008002943-06d1c002b251
 	xorm.io/builder v0.3.7