Accept block for link generator helper methods

[danschultzer]

Resolves #151

Since do block runs before it's passed to the method there is no way of rewriting PowAssent.Phoenix.ViewHelpers.provider_links/2. I don't know if this is the right approach. My general feeling is that this should be solved with pure CSS, or writing out the provider links logic manually.

The view helpers provider very little extra functionality, so it might be best to just direct devs to write them out in their own view modules.