Implement color console for NUnitLite

This fixes #222

I couldn't use the code from nunit-console, because NUnitLite does all it's writing to a TextWriter that is passed in to it,  which may or may not be NUnitConsole. In order to make it work, I split off the write statements inside of ColorConsole and made a new class ExtendedTextWriter, which passes output on to another writer and implements the extended color stuff.

Other changes:
* WriteLabel takes an object as value and does the ToString() itself.
* Color is always disabled unless we know for sure we are writing to the console.
* The command line option to turn off color is not implemented, I'll push another commit with that.