Add an option to ignore files/directories when recursing

This PR adds the `--exclude` and `-x` commands, which allow the user to select a file or a directory to exclude. These can be used more than once to exclude multiple directories, eg.
```
coffeelint -x file.coffee -x subdirectory1 -x relative/path/to/file.coffee -x /absolute/path/to/file.coffee .
```
will recurse through `.` (the current working directory) while ignoring the files `./file.coffee`, `./relative/path/to/file.coffee`, and `/absolute/path/to/file.coffee`, as well as all coffeescript files under `./subdirectory1`.

This fixes #74.