Add ability to use Grunt's files array to specify the files to use.

I was having issues with not having the exclude parameter exposed through
grunt because I am using templates to determine which directory to test, and
need to ignore files based on those templates. Grunt's file src specification
system is much easier to use, so I have added it in here. I added a debug
message to help figure out which files are being included as well to help
users figure out any issues they run into.