Add :proxy option to image filter

I have a site that is served over https. Auto_html works great, but I get a nasty [mixed content warning](http://37signals.com/svn/posts/1431-mixed-content-warning-how-i-loathe-thee) when someone pastes a cat picture from tumblr without using https://

The added :proxy option to the image filter allows the resulting image url to pass through a proxy probably just serves up the image over the SSL connection.