plugin-add and plugin-remove should allow multiple invocations

[Monty Taylor]

In preparation to having multiple config files where options can be given, the options plugin-add and plugin-remove should be allowed to be specified multiple times with the results being additive. For instance:

--plugin-add=md5,rot13 --plugin-add=console should cause rot13, md5 and console plugins to be added to the list. This will allow us to have a directory of config files that can each turn on various plugins.