Fix treatment of standalone tags.

Lines containing ^, /, !, <, or = tags should be stripped from the output if the tags are standalone. The least intrusive way to handle this I could find was to simply "expand" the endpoints of such tags to fill the line - all existing logic will continue to just work.

This fixes 29 spec test failures.

I think all the 13 remaining failures fall into one of these categories:

* Dotted names syntax is not supported.
* Implicit iterators is not supported.
* Partials rendered from standalone > tags should be indented.

I'll might try to tackle the last one next.
Small nit - coding convention in mustache.h/cpp is 'const T&' rather than 'const T &'. This function can also be static since it doesn't read or modify any fields.