Show the current implementation in rakudobrew list
Allow switching to partially named implementations

i.e.:
rakudobrew jvm  # only one jvm build installed
rakudobrew 2013 # matches e.g. parrot-2013.08
Make list() functional -- returns implementations.
Merge pull request #2 from Mouq/list-cur

Better `rakudobrew list` and friendlier `rakudobrew switch`