New features for `perlbrew install`

This pull request fixes a minor bug, and adds two features to `perlbrew install`:

* You can use `stable` or `perl-stable` as the name of a Perl to install, and it gets resolved to the most recent stable Perl (5.even.x)

* A new `--switch` option automatically switches to the newly-installed Perl on completion

The net effect is that newcomers to Perl can be recommended to run `perlbrew install --switch stable` on a new Perlbrew installation, and easily get a fresh, up-to-date Perl environment (even if the instructions they're reading are a year old or more).

Please let me know if there are any changes you'd like me to make.

Thanks for Perlbrew!