Support multiple source extensions e.g. *.coffee and *.iced

Usage: `--extension coffee,iced`

Some projects, for reasons beyond the developers control, may contain multiple coffee file extensions, e.g. coffee & iced-coffee. Specifying multiple extensions will allow parsing of both types of files.

The default of `coffee` has not been changed.

Since explicit CLI tests do not exist we test this new feature via `spec/lib/codo_spec.coffee`.