Added filters option

[bpiwowar]

A simple patch to add the "filters" option which is useful to provide easily custom filters to pug.

[ForbesLindesay]

Thanks for this fix I've invited you to become a team member of the pug webpack team. If you're keen, please accept the invite and then let me know your npm username and I can give you permission to publish releases.

Please still always submit pull requests for any changes you want to make, and allow at leat one working day for other people to review. If you don't get a response within one working day, feel free to merge yourself. If you need my help with something, please "@" me specifically as I don't normally watch this repo very closely.