metaform: new names for standard template sections

metaform: new names for standard template sections

The new convention is to use the `$` prefix for sections perhaps variables that
are global to the document/article. This way you needn't worry about naming
conflicts (so long as your other data doesn't start with `$`.

Specificially, these were changed:

	main_title -> $title
	extra_headers -> $head
	main_body -> $body