[Clopts] Add colours

[Sirraide]

VLC’s --help and -H printouts look pretty nice; perhaps we could ‘take inspiration’ from that. This would entail:

• Adding some way of specifying colours, probably through a single colo(u)rs<...> type. Also add some default themes because why not.
• Adding another builtin option type (like help<>) that adds a --colo(u)r option that allows users to disable/enable colours from the command line. This should probably passed in together with the colour configuration. It would probably be a good idea if all of the colour configuration is handled via the colo(u)rs<...> type, possibly w/ subtypes.
• Actually implementing the option type in question in the printer, as well as precalculating several help messages.
• By default, it should use isatty() to determine whether or not to print colours.
• Add support for 3×256-bit colours (yes, there are ANSI escape codes for that).