New `only` option

Hi @alexch, I've begun to add the new `only` option but I've some issue with it.

The `only` regexps are compared to all dir and file paths listened so with `[/foo/, /.*\.rb$/]` no dir paths will match and nothing will be returned. Problem!

So I would like to add a little more context and maybe add two new options:

- `only_dirs`: will only listen to specific directories (recursively for now)
- `only_exts`: will only listen to specific extensions (so all non-ignored directories are listened by default)

These 2 options could be used together and seems more clear to me.

What do you think? Thanks for your feedback.