Note the upstream refactor from mkdocs-simple v1 -> v2 was fairly large, so
this is a rather larger update than most. And in fact one feature of
mkdocs-semiliterate (universal markdown extraction) was incorporated into
mkdocs-simple, so the initialization of this plugin is noticeably simplified
as a result.
Also updates mkdocs to 1.4.
That is to say, the destination will now be interpreted as an entire
path relative to the source, not simply a filename in the same directory
as the source.
Resolves#20.
