[Commands] Add a (currently empty) `stats` command.

This one will be used to print statistics about lldb sessions
(including, e.g. number of expression evaluation succeeded or
failed). I decided to commit the skeleton first so that we have
a clean reference on how a command should be implemented.
My future commits are going to populate this command and test
it.

<rdar://problem/36555975>

llvm-svn: 328378