Adding support for .eslintignore and .jshintignore (Closes #484)

This adds support for ignoring files and directories. .eslintignore file is JSON formatted file that takes an array of globs.

I'm not 100% sure I like this approach. One this I dislike is that even if the whole directory is excluded, we are still traversing it. But I feel like checking and re-checking exclusion cache all the time is too cumbersome, and would complicate code too much. Also, .eslintignore format is not final, until we decide what we want to do with the format of .eslintrc files.