Allow colour options from REPL

Add the command :colour or :color. Syntax:

:colour on - enable colors
:colour off - disable colors
:colour TYPE OPTIONS - set a colour

where TYPE is keyword, function, type, implicit, boundvar
amd OPTIONS are ANSI colour names, underline, bold, vivid, dull
Tab completion for setting colours at the REPL
Parser fix for colour setting commands
Colourise REPL prompt
Merge pull request #477 from david-christiansen/color-repl

Set colors at REPL