Upgrade to latest version of Discount (2.2.6 currently)

[davidfstr]

(Currently RDiscount 2.2.0.2 is on Discount 2.2.0.)

Should fix the following issues:

• #129
• #130
• #137

Tasks:

• [ ] Upgrade Discount. Make it build locally.
• [ ] Determine new features & fixes from the upgrade.
• [ ] Add new flag bits to RDiscount corresponding to new flag bits in Discount.
• [ ] Test and document each of those flag bits.
• [ ] Make everything build on CI, removing ancient Ruby versions from matrix as needed.
• [ ] Draft release notes and blog post.
• [ ] Publish new version to npm.
• Wait for people to report bugs. ^_^;

[billkarwin]

This has become more urgent because discount 2.2.0.2 has a vulnerability: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-12495

[davidfstr]

discount 2.2.0.2 has a vulnerability

Shucks. I've been disconnected from open source for a few months to take care of some life issues, but I'll put this high on my queue when I'm back.

[fliiiix]

this will be fixed with the #152 branch