`--keep-unreferenced-defs` does not work consistently for all elements

Created by: Ede123

I just noticed that even if I provide the --keep-unreferenced-defs command line argument (added in e01fac82) unreferenced gradient definitions are still stripped from the output.

It does work for other elements, tough, e.g. a <rect>.

I suggest fixing this to work for all definitions or updating the documentation accordingly since intuitively I'd assumed this option to be aimed towards gradients (which is obviously not the case).

Pinging @flosse who submitted the relevant PR #2 so he can have a look.