<rdar://problem/12491387>

I added the ability for a process plug-in to implement custom commands. All the lldb_private::Process plug-in has to do is override:

virtual CommandObject *
GetPluginCommandObject();

This object returned should be a multi-word command that vends LLDB commands. There is a sample implementation in ProcessGDBRemote that is hollowed out. It is intended to be used for sending a custom packet, though the body of the command execute function has yet to be implemented! 

llvm-svn: 165861