Add some hooks for customising user facing templates

[thomasfw]

There's currently no way to customise content/text displayed to end users (mainly on the authorize page).. I know "Howdy" is a WP thing, but I doubt the majority want to address their users like that.

It might also be nice to allow us to choose an application/consumer icon or thumbnail, which can be displayed on the authorization connect page.

I'll work on a pull request.