(see #168 instead) Use hex encoding instead of base64

[timcharper]

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

resolves issue #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 :(

[timcharper]

(closing to open a pull-request from a more specific branch)

See #168