Added [-o <one-liner>] to the "breakpoint command add" lldb command to be able

to specify a one-liner (either scripting or lldb command) inline.

Refactored CommandObjectBreakpointCommandAdd::Execute() a little bit and added
some comments.

Sn now, we use:

breakpoint command add -p 1 -o "conditional_break.stop_if_called_from_a()"

to specify a Python one-liner as the callback for breakpoint #1.

llvm-svn: 113672