Restrict Project Discovery

[nathankleyn]

Allow folders to be excluded from project discovery. Currently, the Travis builds are taking a good long while to run as they are finding the tests in the vendor/bundle directory (which means it's working, but isn't great for build times!) See https://travis-ci.org/shantytown/shanty/builds/64779356 for an example of this.

We need to allow these directories to be excluded, and in the process centralise the globbing process to gain some performance (right now each plugin globs over and over and over again, which is really bad for performance!)