Use hex encoding instead of base64

[timcharper]

The base64 output produced by plantuml does not seem to be supported anywhere

resolves issue https://github.com/skuro/plantuml-mode/issues/166

[timcharper]

We may want to discuss whether it's appropriate to just use base64, or if it'd be better to have this option customizable.

Ideally the base64 format would just work, but I could not get it to work in-spite of my efforts :(