Add plainify template function

To strip away any HTML. May be useful for the .Title in head etc.

People may shoot themself in the foot with this, maybe ...

The replacement function is pretty fast.

content/templates/functions.md

@@ -448,6 +448,12 @@ Runs the string through the Markdown processor. The result will be declared as "
 
 e.g. `{{ .Title | markdownify }}`
 
+### plainify
+
+Strips any HTML and returns the plain text version.
+
+e.g. `{{ "<b>BatMan</b>" | plainify }}` → "BatMan"
+
 ### pluralize
 Pluralize the given word with a set of common English pluralization rules.