'use-data-uris' option should be true by default

[Martín Pozo]

I think that the use-data-uris option is a saner manner of rendering images and therefore a better default.

Thanks and Regards.

[Sytse Reitsma]

I agree. I turned them off by default in this first version, because I wasn't sure about the support for data uris on all platforms.
No-one has complained so far, so I guess I can flip the logic.