Accept filters as an option rather than defined on exports
Accept filters as an option rather than defined on exports

This will fix the fairly common issue where people add a custom filter
to one version of pug but then use a different version to do the
rendering (via something like a gulp/grunt plugin).

pugjs/jade,pugjs/jade,pugjs/pug,jadejs/then-jade,pugjs/then-jade,jadejs/then-jade,omixen/jade,pugjs/pug,pugjs/then-jade,omixen/jade