Added "command history" command to dump the command history.

Also made:
(lldb) !<NUM>
(lldb) !-<NUM>
(lldb) !!

work with the history.  For added benefit:

(lldb) !<NUM><TAB>

will insert the command at position <NUM> in the history into the command line to be edited.

This is only partial, I still need to sync up editline's history list with the one kept by the interpreter.

llvm-svn: 134955