Improve internals of CommandRunner.partition_args()
Improve internals of CommandRunner.partition_args()

- Remove unused local var.
- Add some comments for clarity.
- Only unescape an arg that starts with a colon if the unescaped arg
  would be a command name. Previously, *all* args starting with a colon
  would be unescaped.

wylee/runcommands,wylee/runcommands