Add more plugin hooks

[cryptaliagy]

Right now only a few plugin hooks are exposed, but more could be beneficial, especially for allowing custom actions.

Some basic plugin hook ideas:

• A hook after the TestStep selector is created to add/remove supported test steps
• A hook after the Validation selector is created to add/remove supported validation objects (so things beyond XPathValidation and URLValidation)
• A hook after the OutputValue selector is created to add/remove support output value sources
• A hook for the browser options to allow more fine-grained configuration

More could be added over time, but these would allow plugins to extend Quilla way more substantially