This was the next logical step on the interface revamp for 2.0.
The old API was doing very fishy stuff with allocation: the `rndr`
struct was being called once for every `sd_markdown` call, and the
amount stack space filled with it was just inappropriate (256 bytes
just from the char table and 300 more from pointers in the callback
table).
By creating the renderer on the heap and reusing it, we save a lot of
time on initialization code, at the expense of not being able to
change the enabled extensions on each parse (something which is not
that common anyway).
This will make wrapping Sundown in higher level languages more direct,
given that every other language has OO design, which maps better to
the concept of a reusable renderer.
It is by the goodness of God that in our country we have those
three unspeakably precious things:
freedom of speech,
freedom of conscience,
and the prudence never to practice either of them.
-- Mark Twain, Following the Equator (1897)
