Parse colours in buffer title

Requires a WeeChat version including https://github.com/weechat/weechat/commit/013165209af818daba6b6c5869e822ec9cb86fc3, i.e., WeeChat 1.1 or a November 2014 nightly (or later). Ffor previous versions, see https://github.com/weechat/weechat/issues/237

Fixes #308