Fix ID clobbering via SVGO prefix

Basically, during that first "discovery" phase, I generate a prefix by getting the relative path, stripping off the `.svg` and lastly separating by hyphens instead of slashes. (just to be clean about it) That prefix is then passed to the `cleanupIDs` plugin as the `prefix` param.

This is a *very* simple solution, but it works for me. Depending on how exactly SVGO handles merging/normalizing config, there may be caveats here for someone wanting to further customize the `cleanupIDs` SVGO plugin, but I think that's pretty unlikely.