Support `do ... end` block in view helper

[danschultzer]

Prompted on the slack channel:

Any hint how to change the providers label to an icon in Pow Assent? Do I have to write my self the tag information without the default text?

It would be great if we could inject a tag instead of the text that way it would be great to replace the text

The view helper is really simple, but it make sense to me if you could pass a do ... end block the same way as if it was just a link:

PowAssent.Phoenix.provider_links @conn, do
  tag(:span, provider, ...)
end