Provide alter hook for users to change request options before sent

I'm a little torn about this request. I don't think we want the user to go too far out of the standard but sometimes there are situations where you may want to provide $context for drupal_http_request. 

This also may enable developers to provide a way to provide more dynamic way to set "username" and "password" for things like user-password auth flows by way of the user login, or other, form.

Anyway. Let me know what you think and if you'd rather me submit a patch on D.o i can gladly do that. 

If you are interested in doing github/d.o integration i would also check out the hubdrop project @ http://hubdrop.io/