Allow customizing text on provider sign in link

[joshproehl]

I would like to be able to customize the "sign in with ..." message for a provider. Currently the view helper does not allow altering the message.

I can see two potential solutions:

1. Allow passing a msg param into authorization_link/3
2. Create an authorization_url() function allowing the user to get the raw URL and create their own link.

Option 2 would allow me to add an img tag to the link easily, which would be a nice feature. It would also allow generation a provider URL easily to automatically send a redirect to that URL, which would help in the use case of "use provider only". That may be a different ER which is better executed in some other way though. :-)

[albertmoravec]

I would appreciate having a similar API to Phoenix.HTML.Link where you can pass a do block that gets passed down as the link's content.

Edit: Looks like this is being addressed in #153, but it's still not merged sadly.