Have regal lint fail when unknown category or rule is provided via --enable/--disable options

[anderseknert]

regal lint --disable-all --enable foo .

Should have regal fail loudly unless the foo rule was actually found. Currently it'll just report this as a success, which while technically true, is not likely to be what the user wanted. Note that we'll also need to take custom rules into account, so this is probably a check we'll have to do some time into initialization.