Include local images with base64 encoded data uri

In order to make the generated presentation even more self-contained, this PR allows the user to specify an option to encode images present locally in base64 format and inserted as data uris in the presentation instead of a normal src reference which forces to always have the images next to the html presentation.

This works, for the moment, only for images that are on the machine where cleaver is run, and specified with a relative path.