mdjl testing

[rsrock]

I've tried out the mdjl branch now, and it looks like it works quite well. One issue is that quite a few characters are escaped that don't need to be, but it doesn't affect what's rendered. An example is here, the diff shows the mdjl branch vs. the old pandoc way of doing things.

https://github.com/timholy/Images.jl/commit/ed1afa2c0a74e1f21cf44533b9ecb1a208859f83

Also, I noticed that the timestamp doesn't show up.

Thanks, I had no idea that it was possible to ditch pandoc for this!