List filenames and line numbers when collisions occur

As discussed in #2, grunt-colorguard now shows the filename and line number of the CSS file where a collision occurred. This is ideal for projects that are using a bunch of separate stylesheets in development that are being `import`ed together when served in production.

Collision alerts now look like this:

    >> #FACADE [testing3.css:2] is too close (0.3239970889503257) to #FACADD [testing3.css:14]
                ^^^^^^^^^^^^^^

Keep in mind that _grunt-colorguard is looking at raw CSS only_. If you're using LESS, SASS, or some other preprocessed CSS language, these filenames and line numbers won't match up with your source code. You'll want to look up the offending line in your generated CSS.