Support for color palettes for undefined author colors.

**Is your feature request related to a problem? Please describe.**
By default, the commits pane shows initials of the committer in a set of randomly chosen colors. I would like to have these colors chosen from a list of pre-determined colors so that they may match the rest of my color scheme.

**Describe the solution you'd like**
I would like a new option in the user config to allow the user to provide a sequence of colors and have them be chosen from for any author names which have not already been specified. E.g.

```yaml
gui:
authorColors:
'John Smith': 'red'
'Jane Doe': '#00CC00'
'*':
- 'red'
- 'green'
- 'blue'
```

This feels like an extension of the capabilities of the current wildcard syntax (since the current behavior is equivalent to this behavior provided a color palette of length 1). Hence why I put it under that key in that example.

**Describe alternatives you've considered**
The closest behavior I can get to this is doing it manually/via a script with the existing `authorColors` feature. That is, going to every unseen committer, hashing their name to get it from my color table and updating my config with it. But for a repository with hundreds of developers that gets unmaintainable.

**Extra Context**
I wanted to get some thoughts on this idea before I dive into making a PR. So feedback is welcome!