Added a views generator

[sandergroen]

Added a view generator and documentation. To make it easier for users to modify those views.
This closes #46, closes #46, closes #173, and closes #183

[takahashi64]

Hi @sandergroen, I tried to use this to generate a view to override the default view for Devise::PasswordExpiredController#show

This was the output:

  invoke  DeviseSecurityExtension::Generators::SharedViewsGenerator
  exist    app/views/devise/password_expired
  identical    app/views/devise/password_expired/show.html.erb

However, the default view is still being rendered. I encountered the same problem when I tried to do the above without your generator, so I thought you might have some expertise on this. Thanks.

Edit: see my post on stackoverflow

[oscaralanpierce]

This PR would be so helpful for us!