Keep groups by id

[Artoria2e5]

It is useful at times to want to keep some groups while allowing svgcleaner to go wild on the rest. A "no-ungroup" list might be handy in these situations.

Having a selective ungroup may also help with the implementation of <switch> support. Currently I handle these sort of files by manually commenting out the element and the attributes with ungroup turned off, but that's suboptimal.