[Engine(|-Tests|-IRC)] Add formatting capabilities to MessageBuilder.

Allows construction of messages from strings, nicknames, message parts and other
messages using String.Format-like format strings. Additionally, modify the IRC
engine code to make use of this feature, giving translators more control over
the presentation of notification strings.
Merge pull request #59 from RavuAlHemio/experiments/msg_append_parser_2_electric_boogaloo