Updated getAuthorizeURL in twitteroauth.php

Added the ability for users to pass URL params to the authentication URL, such as force_login and screen_name.

You closed @miteshashar's abraham/twitteroauth#111 awhile ago, but this code change is different in that it leaves it open ended, for future Twitter API changes and parameter additions.

I want `getAuthorizeURL()` to append my URL params - I don't want to get the URL and then have to add the params on the function return. Currently the function returns `?oauth_token=`, but what if some day it doesn't? I don't want to have to parse the function return (`$authorizeurl`) to see if there is an existing param, and then use `?` or `&` - a lot of extra work that the function can just do itself.