Dynamically generate appropriate ansi-term colors

When an `ansi-term` buffer is launched, it gets the standard ANSI colours.

This can be overridden by explicitly setting `ansi-term-color-vector` when `term` is loaded. This replaces the standard 8 ANSI colours with the colours from the vector passed in.

I have two concerns with this commit:

1. The code isn't great - really the fetching/naming of colours wants to be factored out instead of being generated in a giant `flet/let` construct. This way the vector could be generated outside the main construct;
2. I'm not sure completely clobbering `ansi-term-color-vector` is the best thing to do, but it's what [other themes][0] do.

[0]: https://github.com/bbatsov/zenburn-emacs/blob/3672f48b8b8dad7521ef1f4b075a327e735e03f0/zenburn-theme.el#L583