Support for base64 data

As reported in issue #402 added a conditional to not set cross-origin setting when loading data-uri images. these will be loaded locally so we shouldn't need this for that file type.

I tested loading an external JS file which had a data-uri set as a variable and used that variable in a p5.js sketch which was running on a different server. Loading and using the external data-uri var didn't cause an issue — so I think were good with that edge case.