Make keyword against current Mercurial available as standalone extension

Branch off backwards compatible version The default branch will contain the version without the overhead for backwards compatibility. This will make a standalone extension available which works with current Mercurial.

Avoid name conflict with dispatch package(s) Thanks to Marcin Kasperski for spotting and analysing this. Message-ID: <87wsswvhbk.fsf@cauchy.softax.local> ff.