Colorize log messages, even under Windows

I basically used logbook’s machinery, with one important change:

> If you are on Windows, the call to init() will start filtering ANSI escape
> sequences out of any text sent to stdout or stderr, and will replace them
> with equivalent Win32 calls.

This also allows me to make `install_theme` and `install_plugin` output
colorful code under Windows (besides, it was broken and produced garbage with
ANSI sequences under Windows and plain text everywhere else, sorry)

Note that this requires further testing under all platforms.

Supersedes #1044.

cc @asmeurer @ralsina