Provide a function that lists supported providers

[alex]

Here's how it currently works:

# Map URLs to handlers
Route('/auth/<provider>',
         handler='handlers.AuthHandler:_simple_auth',
         name='auth_login'),
Route('/auth/<provider>/callback', 
         handler='handlers.AuthHandler:_auth_callback',
         name='auth_callback'),

Nice tip from @erichiggins:

Perhaps a convenience function could be written that simply supplies a list of
supported providers from the app's config file/dict. Then folks could make the above
route expression(s) programmatically.

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Moved here from https://code.google.com/p/gae-simpleauth/issues/detail?id=5