feat: allow nil for languages

[benlubas]

Allows you to pass nil to activate() for languages. When languages is nil, it's set to all the languages that otter supports.

Currently if you pass nil it just errors out, and there is no easy way to just "enable all the languages"

Bonus, there's no errors when you don't have a language server configured for a given language.

[jmbuhr]

Perfect, I think with my recent changes this should then only activate the languages actually found in the document.