ignore `node_modules` by default

[masterkain]

https://github.com/guard/listen/blob/master/lib/listen/silencer.rb#L6

what do you think about adding node_modules to the default list?

[ColinDKelley]

@masterkain Is that a common folder to have lots of changes that typically don't need to be listened to?

[thibaut-pro]

Yes.