another take on allowing one to install several perl variations (and now also versions) on one single command

(follow up to pull request https://github.com/gugod/App-perlbrew/pull/333)

I have rewritten the feature from scratch as the new command `install-multiple`.

Now besides the `--both` flag it also accepts several perl versions so one can write:

     perlbrew install-multiple 5.6.3 5.8.9 blead --both thread --both 64int

There are also two shortcuts: `--common-variations` and `--all-variations`.

Besides that I have made some small changes here and there in order to eliminate a couple of bugs, remove duplicated code and allow for a simpler implementation of the install-multiple command.