Exporting of extensions

[goodmami]

Support for reading lexicon extensions will be in the next release, but I'm creating this issue because the exporting of extensions probably won't be released at the same time. The problem is this sort of turns export on its head, requiring it to traverse things like sense relations and forms first to see if they are linked to a sense, lexical entry, etc. that is not in the current lexicon and, if not, create the appropriate external entities. This switch from top-down to bottom-up export will involve some effort which would delay the v0.6.0 release, so it'll have to come later.