Added load_url for updating gif data
Added load_url for updating gif data

There's no equivalent to changing src, so I made some fixes to init and
load and added load_url(src, callback). Now gif data can be swapped with
a new url without having to make several changes to the DOM

doclements/libgif-js,ToadKing/libgif-js,buzzfeed/libgif-js,paulvi/libgif-js,sibnerian/sibgif,ToadKing/libgif-js,paulvi/libgif-js,buzzfeed/libgif-js,doclements/libgif-js