Support RGB text and background colors

Up until now, we have only supported the 16 colors allowed by ANSI escape
codes.  This patch adds support for RGB color escape codes as described in

https://projects.kde.org/projects/kde/applications/konsole/repository/revisions/master/entry/doc/user/README.moreColors

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>