Rework the alias interface to support the following new features:

bk alias -h # shows what aliases are "here"
new automagic aliases: HERE and THERE in addition to ALL and DEFAULT
which are now reserved in all cases (DeFaULt)
plug revisions into aliases so that when we clone -r we get the
correct version of the aliasdb (as of the rev we're cloning).

 — Implementation Details

STUFF THAT’S NOT DONE OR NEEDS TO BE CHECKED

one idea is: