Add $regexp custom $options support

With optional support for [XRegExp](http://xregexp.com/) to support additional modes supported by MongoDB:
- `x`
- `s`

Start by locating the $regexp query handling and the optional XRegExp integration point; the issue identifies x and s as the additional modes to investigate. Done means custom $options support covers both modes while preserving existing regular-expression behavior, with the relevant test coverage updated.

Written by the indexing model from the issue text.