Add support for Dropbox and Cloud links

[gkoberger]

@clouserw requested and @cvan said he'd make it happen.

For cloud links, adding anything after the slash will make it an image:

http://cl.ly/image/1H2B2C3b1s3w -> http://cl.ly/image/1H2B2C3b1s3w/test

Not sure about Dropbox

[rik]

@clouserw or @cvan Do you have a dropbox URL we could work with to test?

[cvan]

https://www.dropbox.com/s/ewqohutuiqzk6us/Screenshot%202014-09-08%2016.38.04.png?dl=0
https://www.dropbox.com/s/ewqohutuiqzk6us/Screenshot%202014-09-08%2016.38.04.png?dl=1