Implements a pluggable scrollbar model. Interface not documented yet,
but quite simple (see forthcoming simplescrollbars addon)

And manually compensate for our own scrollbar sizes.

This way, we're no longer dependent on the size of our scrollbars
being the same as the size of native scrollbars, and can start to
think about custom scrollbars.

a wiki engine

The mime type is currently accounted for in the htmlembedded definition, but erb
is not listed in the `mode/meta.js` index.

Webkit can't deal with them.

Issue #2945

When closeOnEnter is disabled the enter event never gets default
prevented this causes issues for IE9/10, this way implementors can
optionally preventDefault if they want

Issue #2915

- more style classes: operator, builtin, meta, brackets
- literal metas (@ and ^^)
- many more operators
- mime type and extension from spec

See http://discuss.codemirror.net/t/loadmode-js

This prevents a stray $ from setting the whole rest of
the document to math mode.

Closes #2928

I'm rewriting the way that visual mode is written to:
1. Fix some tricky bugs, like https://github.com/codemirror/CodeMirror/issues/2913, and a couple I uncovered while doing this. (Unit tests updated)
2. Significantly simplify blockwise visual mode

The core of the changes here is that I'm creating a vim selection model. The selection model consists of

```
vim.sel.head
vim.sel.anchor
vim.visualMode
vim.visualLine
vim.visualBlock
```

I'm creating a 2-way binding between the vim and codemirror selection models. All motions now rely only on the vim selection model. Operators and actions still rely on the codemirror selection model, since they operate directly on the text.

Sorry for the gigantic change but I couldn't make it any smaller and still pass all the tests... I'm planning to delete `getSelectedAreaRange`, `getLastSelectedAreaRange`, and `selectBlock` in future changes as they are obsolete now that vim keeps its own selection model.

An added bonus here is that visual line mode now has a cursor that can be moved.

Add tests

Also add support for "gutter" in add/removeLineClass.

Issue #2920

Issue #2915

Issue #2919

The mode associated with a token is now the inner mode at
the time when the token is started. The htmlmixed and markdown/gfm
modes were fixed to return empty tokens during mode switches
when necessary to enforce this rule.

Issue #2919

Defaults to `window`

Issue #2922

Issue #2921