Chatterino2: Add filters to channels.

[Mm2PL]

Describe the solution you'd like

I would like for each channel to have filters, a search string that is matched to all messages. It could be specified like channel_name filters filters filters.... The channel change popup could be changed to make the filters a separate input box.

Describe alternatives you've considered

not doing it

Additional context

It would be pretty easy to reuse the existing search predicate system.

[cinemassacres]

https://github.com/Chatterino/chatterino2/issues/228

[fourtf]

there's another issue I think (and a pr)