Allow adding custom highlight groups that are not in theme.lua

[mawkler]

This PR added the ability to customze highlight groups, which is awesome and a drastically better way of setting highlight compared to using the builtin highlight command. However onedark.nvim currently only allows customizing highlight groups that are already defined in theme.lua, and not new highlight groups that don't yet exist in theme.lua. Would it be possible to allow adding any highlight group when calling require('onedark').setup()?

[ful1e5]

Now you can add a non-exists highlight by enabling dev mode.

[mawkler]

Thank you for implementing this!

Just out of curiousity, what is the idea behind dev mode and why is this feature only available in dev mode? :)