Add optional config for custom filter (and standard js linting)

[billymoon]

Add custom filter option so that as a consumer, I can control which files have routes created for them.

My specific use case is to only add routes for index.js files so that other shared files can co-exist in routes folder alongside routes. I was making a service that had some shared code between routes, and was forced to move that code far away from the service I was writing which I did not like.

... and a small bit of housekeeping, adding standard js conformity to code

[coveralls]

Coverage remained the same at 100.0% when pulling d15d317 on billymoon:master into 4492000 on jesseditson:master.

[jesseditson]

Looks good! I'll land and publish.

[jesseditson]

Landed as v0.3.0!