Add basic functionality for user highlights

[Bauke]

This adds the basis for highlighting all messages from a specified user, I'm not sure if we want to separate phrases and users entirely so for now I've kept them under the same array (settings.highlights). If this were to be added I think we'd want to specify how to highlight users too in the settings panel.

Closes #388

[scsddeputy]

i think it would probably be easier to just have another array in the settings to set usernames to highlight instead of making it part of the word highlight function

[kumpelblase2]

In reference to my comment, I personally would be fine with keeping a prefix and the same list of highlights. Just the usage of regex in this particular instance might be going a little overboard.

Do we have any reference on how this is done in other, similar applications? I'd lean towards doing it similar to that, no matter if it's more common to just have one "highlighting list" or multiple. As one reference, BTTV just has one list (while different brackets around elements denote what kind of highlight it is)

[Pnoexz]

Hey, I've been working on a similar but opposite feature: don't highlight from certain users. Would be mostly used for streamers to ignore their bots, but could also be used to "soft ignore" people. I've also run into the issue of where to add this setting. Maybe we can marge both features in a "Users highlights" setting.

[mccxiv]

I'm still outside the country, will be back in a week to catch up on PRs

[mccxiv]

👍