Add support for ansi named colors

[Misterio77]

Hey!

I frequently change my terminal colors, and setting RGB colors for my todo folders often leads to unreadable text.

I thought it would be nice to offer an option for setting one of the usual 16 terminal colors, in addition to the 24-bit rgb ones. So, i've added support for them.

It works for all the 16 ansi colors (including the 8 bright ones).

This possibly fixes #297

Here's how it looks like (using dracula scheme, and colors red, cyan, and bright purple (which my terminal renders as bold purple) for the folders):

[WhyNotHugo]

I've mixed feelings here. I like the idea, but this breaks compat with the vdir format. Two actual issues are:

1. vdirsycner syncs this to CalDav servers. I'm not entirely sure they support this format. It might be worth just trying, but if it doesn't work syncing to servers and other clients, that'd be a dealbreaker.
2. khal will fail to parse these and stop showing colours. I use khal myself, and other todoman users do too. Parallel support in khal could be required.

[Misterio77]

Thanks for the feedback.

I wasn't aware the color setting was something standardized. I guess my provider doesn't support syncing colors.

Khal has a config option for manually setting colors, so I guess something similar could be implemented here, would that make sense?