This adds a "batch mode" to lldb kinda like the gdb batch mode.  It will quit the debugger
after all the commands have been executed except if one of the commands was an execution control
command that stopped because of a signal or exception.

Also adds a variant of SBCommandInterpreter::HandleCommand that takes an SBExecutionContext.  That
way you can run an lldb command targeted at a particular target, thread or process w/o having to
select same before running the command.

Also exposes CommandInterpreter::HandleCommandsFromFile to the SBCommandInterpreter API, since that
seemed generally useful.