Implement --self-upgrade capacity.

As from now, to install a new version of pgloader when you have an older
one, say because there's that bug that got fixed meanwhile, all you need
to do is run

  $ git clone https://github.com/dimitri/pgloader.git /tmp/pgloader
  $ pgloader --self-upgrade /tmp/pgloader <options as usual>

Any Common Lisp developper using the product is already doing that many
times a day, it might prove useful for users to be able to hot-patch
themselves too, after all.