Allow CsrfProtect(blueprint)

[fayazkhan]

Selectively enforcing CSRF is preferable rather than csrf.exempt(blueprint) IMO.
Is it doable? (I was looking into it and was lost at what to do with the config variables.) Does it make sense? Is #111 really the "obvious way to do it"? (I'm not Dutch)

[lepture]

With this change e85808b

You can set WTF_CSRF_CHECK_DEFAULT to False, and protect a single blueprint with:

csrf = CsrfProtect(app)

some_blueprint.before_request(csrf.protect)

[fayazkhan]

That's good enough I guess.