Make parser codebase of vimwiki be optional feature

[chipsenkbeil]

Rather than always bringing in the parser code, which includes dependencies like nom, bytes, and bytecount, it would be nice to make that entire section optional. This would be useful in scenarios where other tooling wants to either a) write its own parser, b) build vimwiki elements by hand, or c) load up vimwiki elements that were serialized elsewhere for it to operate on.