Add support for custom style highlights/annotations #140

This feature will require changes in Artemis4J for the grading config.

The goal is to color code different groups/kinds of annotations to make them easier to distinguish.

IntelliJ supports highlighting by underlining code. This could be implemented at the same time, allowing highlighting that is less pronounced.

What would be necessary in intelligrade:

• Read color suggestions from Artemis4J config like HighlighterColor.RED and then translating it to a hex color that works well with the theme (either light or dark)
• Allow configuring the colors in the settings?