Allow cmsDriver.py to accept multiple --customise options

To make it easier to globally modify the behavior of IB RelVals generated
by runTheMatrix, we modified cmsDriver.py option handling to allow it to
properly handle multiple --customise options. The previous behavior allowed
multiple --customise but ignored all but the last one.

This change was tested on
- no --customise options
- one --customise option
- multiple --customise options
- one --customise option with multiple arguments seperated by ','