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.

Damaged in 1a56b954

Issue #3516

Since foo:bar was matching as a regex (as reported in #2521 but still
occurring for me), limit URI schemes to those in the CommonMark spec
for autolinks http://spec.commonmark.org/0.22/#autolinks

Closes #3386

Closes #3345

Issue #2903

after header immediately followed by a code block, GFM formatting is
broken (SHA, #issue, URLs).

overlay.codeBlock gets it wrong (false inside fenced block, true after).
Not yet sure why.

Bumping them for every release just adds too much git
noise and history blowup.

Added `combineTokens` for overlays that can override default
`combine` argument.

Closes #2015

Closes #2039

Closes #961

Closes #1165.

Issue #1160

Tests are now specified in a less redundant format, and should be
easier to read and write.

Closes #1133

Fenced code blocks *do* have syntax highlighting.

This ensures that markdown.js is loaded when
gfm mode is loaded using CodeMirror.autoLoadMode

As spotted by the extended linter.

- Add options to Markdown mode to make it easier to extend
- Add GFM text overlay
  - SHA, User@SHA, and User/Project@SHA
  - #Num, User/#Num, and User/Project#Num
  - Vanilla URLs
- Add GFM-specific tests
- Fix overlay code so blankLine() is called
- Fix GFM for v3

This makes the formatter easier to adjust to new mode, and
cleans it up somewhat.

Add CodeMirror.defineOption function for defining new options.

Add findMatchingBracket to the methods exported by this extension.

Closes #746