Add a way to pass arguments to mustache helpers (php and js)

right now - a helper will only receive the block of text from a section.

Handlebars, (and others) allow helpers to receive arguments - e.g.

str error filenotfound
vs
#str}}error filenotfound{{/str

if we want to implement a conditional helper it will require this
e.g.
#if contextvara "eq" "5"
/if

This allows us to have presentation logic in the templates, which will make the templates more re-usable.