Implemented runtime loading of chat-based server commands

-Created ChatCommandAttribute class and applied it to existing
implemenations of the CommandBase class.
-Created static constructor for Packet11ChatSend class which reflects
over compiled assembly and loads valid classes which have applied the
ChatCommand attribute.
-Changed Help command to rely upon data populated during runtime load of
the Packet11ChatSend static constructor.  Content is now dynamically
loaded in the same way as chat command keywords.
-Changes in the way pass-through commands work, and simplified method of
adding future Starbound keywords.