Use custom highlight groups

[b0o]

Currently, highlights are hard-coded to built-in highlight groups. This means users can't customize the appearance of the Other.nvim specifically, it might look quite bad:

It would be better if there were Other.nvim-specific highlight groups so that users could override them.

[rgroli]

@b0o Great suggestion, I implemented two custom hightlightgroups. OtherSelector and OtherUnderlined