Support regular expressions, e.g. for using in the 'replace' filter.

Syntax: `r/BODY/FLAGS`.

Example: `{{ 'ABa' | replace(r/a/gi, 'z') }}` will render `zBz`
Re. `instanceof` - there was no particular reason, but I'll change it to that if you prefer?

`maxCount` has a sort-of equivalent in the `g` (global) regex flag.

I'll rebase this on master to resolve the conflicts right now.