Closes #4628

As per http://spec.commonmark.org/0.27/#html-blocks

Issue #4611

Closes #4568

Issue #4528

Closes #4528

Closes #4484

Issue #4426

As per CommonMark (conflicting with markdown.pl, but never
mind markdown.pl)

Closes #4395

- Images now come with tons of information to style them. This mainly
  enables to style alt-texts and potentially make them look like links.
  While still differentialting between links and images.

Closes #4082

Issue #4013

Closes #4013

Closes #3994

Closes #3981

There were two issues:

1. Github flavored markdown calls Objective C `objective-c`
2. The gfm.js mode didn't include `-` in its fenced code block regex

Additionally, test/index.html did not import the mode/meta.js file,
which prevented mode lookup by name from working in unit tests.

See http://spec.commonmark.org/0.24/#list-items

Closes #3833

Rather than hard-coding the bare XML mode.

Issue #3829

Closes #3800

Closes #3787

Closes #3777

Example: `[foo\[bar\]foo]: https://example.com`

The `markdown` mode now supports styling overrides. The configuration
object passed to the class now accepts a `tokenTypeOverrides` object,
which allows the user to sepcify different CSS classes for elements.
This is useful for the case where the default CSS classes are reused
for multiple elements (e.g. `list1` maps to `variable-2` by default)
and more customized styling is desired.

Unit tests have been added to cover all types of styling overrides.

Issue #3496

Issue #3535

Closes #3343

Closes #2962

Closes #3386

More than once people have asked questions expecting markdown mode to
support GFisms and were simply not aware of gfm mode existance.
Hopefully this will make it more discoverable.

Closes #3384

Issue #3325

Issue #3321