Add '--extensions' option to use in PHPCS and for allowed extensions

[LetrixZ]

This PR adds the --extensions option to pass to PHPCS command and to use if for allowed extensions when a dir is used instead of a file.

[sirbrillig]

This looks very cool. Thank you for the effort! There's a few failing tests, but I think they just need the new argument provided (try composer precommit to run everything). Could you also add this option to the README?

[LetrixZ]

Done.
Passed CliOptions instead of the single option to avoid type issues.

[juanmitriatti]

Hi @sirbrillig
This PR is really necessary and useful, will you merge it with the current beta release?.
Thanks

[sirbrillig]

That is my plan, yes. I just have had a busy weekend.

[sirbrillig]

This looks great! Thank you!