Optional PHP filtering and Jade options fix.

I recently discovered your Brunch plugin and find it very useful!  I'm starting to use it to help with making Drupal themes so I made some changes to help with that.  Hopefully these changes are also generally useful.

Here's what I did:
- Added the option to use PHP filtering.
- Added the option to change the extension used for static file output.
- Fixed Jade options not being used.

New options:
- **filterPhp** -- Boolean (false by default).  Use the PHP custom filter.
- **outputExtension** -- String ('html' by default).  The extension to use for the output file.

The PHP filter that's used:
https://github.com/viniwrubleski/jade-php