Colorize output

![The new look](https://dump.tribut.de/weechat-otr-color.png)

 - 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 (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.
 - Better ideas for the (default) colors/prefix?
 - Would you rather only highlight specific words ("authenticated") or is that much color ok?