Color REPL under -Dscala.color

We already use -Dscala.color when using -Ytyper-debug

This tries to reuse the colors chosen from the debug flag:

* Bold blue for vals (e.g. "res0")
* Bold green for types (e.g. "Int")
* Magenta for the shell prompt (e.g. "scala>")
Add color to severity in REPL reporter

* Errors are red
* Warnings are yellow
Use color REPL after writing a def
Merge pull request #3993 from puffnfresh/feature/color-repl

Color REPL under -Dscala.color