Add support for pluggable views

[jcass77]

To make stormpath-flask truly customizable, we probably need to allow users to override the Flask views in addition to the HTML templates (which is already supported).

This would most likely involve converting the current view functions to pluggable class-based views, which would be much more flexible and make it easier to enhance some of the current views and WTForms.

[rdegges]

Agree 100%. This has been on the todo list for quite a while now.