Colorize output

This has become somewhat of a general "rework output" branch.

![Demonstration of the new look](https://dump.tribut.de/weechat-otr-colors.png)

I would be grateful for feedback specifically regarding the following points:
 - Does everyone like the proposed default colors? The default prefix?
 - This branch only colors whole lines (with the exception of "unencrypted message received"). I was considering only printing the word "authenticated" in green for example, but the approach used here allows for cleaner code and I have no specific preference regarding appearance.
 - I decided to make the prefix configurable after all. How easy a specific username is being mistaken for our prefix depends a lot on general weechat customization (see `look.prefix_same_nick`). Also, custom prefixes are easy to implement and allow parts of the prefix to have different colors (for why this is important see the comments in #51).
 - Tests are very basic. My implementation basically ignores colors in the output because `weechat.color` always returns an empty string. Also, `weechat.string_eval_expression` doesn't modify the input. I think that's fine actually.
 - The classification of specific messages: Do you agree with my decisions?