Improve Imgur URL parsing

Now support for the following of Imgur URL formats:
* Normal Imgur URL: http://imgur.com/xxxxx
* Imgur Album URL: http://imgur.com/a/xxxxx
* Imgur URL with /gallery: http://imgur.com/gallery/xxxxx
* Direct image URL: http://i.imgur.com/xxxxx.jpg
* Mobile Imgur URL: http://m.imgur.com/xxxxx
* Secure Imgur URL: https://imgur.com/xxxxx
* (Non-standard) Normal Imgur URL with extension: http://imgur.com/xxxxx.jpg
* (Non-standard) Direct Imgur URL without extension: http://i.imgur.com/xxxxx
* (Non-standard) Direct Imgur URL with weird extension: http://i.imgur.com/xxxxx.jpganyextensionisallowed
Only add "obey_over18" parameter for Reddit OAuth request