Update markdownfmt to use blackfriday v2

[jsternberg]

Fixes #39.

[dmitshur]

Thank you for sending this @jsternberg. I did not forget about this, but it will likely take me a bit of time before I can get to reviewing and merging this. I hope that's okay.

A part of the complication for me is that I'll likely want to sync the upgrade to blackfriday v2 for my other package github_flavored_markdown, so they both use blackfriday v2 at the same time.

The reason for that is because I use both of them in frontend code, and having one import blackfriday v1 while the other v2 would mean my frontend code size would increase.

An alternative I will consider is to have two versions of markdownfmt, one that continues to use blackfriday v1, and another that uses blackfriday v2.

[jsternberg]

👍 for anyone who needs the library form for their own code like I do, they can just use my fork.