specify arrays of globs for test files, instead of a single `dir` string

[jokeyrhyme]

Really enjoying your work so far. Keep it up. :)

With the current dir functionality, we have to make the following assumptions:

• all the tests we want to run share a common parent directory
• we actually want to run all the tests found beneath that common parent directory

I have a use case where this isn't strictly true, and these assumptions may not be correct with PSR-4 components.

I would find it useful to be able to specify precisely the test files I want to be executed, as an array of string paths. It would be especially useful to be able to provide string path patterns, as with the src setting in grunt-jslint.