add ignore option

[joshwnj]

Hi, for a project I'm working on we sometimes have non-js files going through the browserify pipeline. To avoid a parser error I'd like to add support for opts.ignore.

Thanks

[lgersman]

:+1:

i just wanted to use brfs in conjunction with css-modulesify (css-modules, yeahhh !!) and failed also because of the missing ignore option in btrfs.

[jamescostian]

:+1:

[Quramy]

:+1:

[eugeneware]

I also wish to use with css-modulesify.

[eugeneware]

I've published a temporary package with the ignore option merged in until this gets merged - https://www.npmjs.com/package/brfs-ignore