Formatter pretty colors param

[ltickett]

Adds support for custom colors in the formatter-pretty plugin.

Closes: #321

[ltickett]

I couldn't find any tests for the current formatter to be able to add tests for my changes.

[DavidAnson]

Thanks! Tests run as part of the base project. The scripts "ci" and "update-snapshots" will be helpful here.

Regarding the change itself, as discussed in the issue, I would much prefer an approach that tweaks the single problematic color. It sounds like you think that is possible. What's done here is definitely more flexible, but commits to all of those specific fields and adds a many opportunities for the code to crash if someone doesn't know what colors are valid or misspells one. It converts this fairly safe and easy formatter into something potentially difficult to use.