Closes #3470

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.

Fixes #3449

And remove some unused state properties

* Fix bug where you could paste into a `readOnly` editor
* Show a cursor at the drop location when dragging over the editor
* The [Rust mode](http://codemirror.net/mode/rust/index.html) was
  rewritten to handle modern Rust
* The editor and theme CSS was cleaned up. Some selectors are now
  less specific than before
* New theme: [abcdef](http://codemirror.net/demo/theme.html#abcdef)
* Lines longer than [`maxHighlightLength`](http://codemirror.net/doc/manual.html#option_maxHighlightLength)
  are now less likely to mess up indentation
* New addons: [`autorefresh`](http://codemirror.net/doc/manual.html#addon_autorefresh)
  for refreshing an editor the first time it becomes visible, and `html-lint` for using [HTMLHint](http://htmlhint.com/)
* The [`search`](http://codemirror.net/doc/manual.html#addon_search)
  addon now recognizes `\r` and `\n` in pattern and replacement input

Closes #3456

Closes #3457

Should reduce the indentation issues after long lines

Issue #3443

- Provide custom list through CodeMirror hintOptions to kickoff
  autocompletion for domain specific usage.

Since the library is completely oblivious to module loaders

Issue #3447

For example:

body
  color $red

.content
  color #000

The .content was highlighted as the variable-2

Issue #3438

Closes #3417

Issue #3434

Also some general improvements to material.css and solarized.css

Don't treat backslashes specially inside of them

Closes #3428

Issue #3423