Revert "Revert "Add rgbToXterm unit test.""

This reverts commit c7089f1cb360010b18e6396757c99df87ee875c9.

Conflicts:
	Dapper.d
Minor stylistic thing.
rgbToXterm unit test enhancements.
Another unittest fix and more stylistic improvements.
Remove alias this.
Add opCast to FormattedString.
Replace merge() with opBinary!"+"().
Update readme.