Bugfix: Return empty properties when no player

Gracefully handle property access when no player is found by returning
an empty property instead of segfaulting.
Add methods to print metadata

Adds methods to the player to easily print artist, title, album, and
arbitrary properties of the metadata object.
Add metadata KEY option to cli

Pass KEY to only print the value of that key of the metadata. Special
cases for "artist", "title" and "album" do not require xesam prefix and
apply basic formatting.
Bump to version 0.2.0