Make it actually work, document it, declare the option,
make sure the code runs on pre-ES5 browsers.

Issue #3550

Issue #3525

So that the xu/msgenny modes can be dynamically loaded from meta.js

Issue #3523

Closes #3512

Issue #3511

Issue #3487

Issue #3462

* 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

... and export (via defineExtension) a method to explicitly trigger
linting from user code.

Issue #3395

Issue #3395

Issue #3387

Issue #3372

Closes #3348

Issue #3346

Original code by Evan Czaplicki