Added some accessors that allow toggling certain behaviors

First of all, thanks for this library. It's great.

We recently started using it in place of OptionParser for a certain bit of functionality. However, some of the default behaviors were a little too rigid for our use case, so we added some accessors that would allow toggling of some of these behaviors. My belief is that these changes should be transparently backwards compatible.

We also added support for registering callback blocks that are triggered when the parser encounters an option.

We'd love to see this get merged in to the mainline so that we aren't managing a forked version :) Let me know if you think that might be possible!

via [gitorius](https://gitorious.org/trollop/mainline/merge_requests/9)