Old-school terminal color support

[benjiwolff]

Is your feature request related to a problem? Please describe.
When I open neogit in my macOS Terminal.app, there are no colors to help me out.

Describe the solution you'd like
I would like to have appopriate ctermfg colors assigned to each highlight group defined by neogit, such that there is some visual clarity in the terminal (see draft #1711).

Describe alternatives you've considered

• Add a disclaimer, that highlights are not supported for old school terminals and list terminal alternatives

Additional context
I was completely unaware of my terminals color limitations until I started using neogit because all other git tools I use (e.g. git cli, gitsigns) display colors correctly. I expect others to be confused as well.