Split modify-option-group into add/remove commands

See #214 for a description.

This also included a few changes to the customizations code:

* Allow a param to have an external client set its name (used for arg renames).
* Add utils module to make encapsulate arg renames.
* Use the names from the command/arg table for the event names (to accommodate changes to the command/arg tables)
* Use the command table name for the doc name.  This is so that "aws foo opname help" uses "opname" in the man page/html docs)