Issue #2494

Add getHelpers and registerGlobalHelper, allow modes to specify arrays
of helper names, support modeProps in mode specifications.

Update foldcode and show-hint to make use of more powerful helper
system by defining an `auto` helper that tries multiple helpers, and
using that as default.

Move hinting functions into it.

Closes #1133

Should help trigger #836, and is cleaner anyway.

Closes #840

Fixes a problem where the editor would always be 30 pixels smaller
than its set height.

The height is now set on the outer wrapper DIV again, and some
height: 100% tricks are used to make height information flow
both ways (inside-out in demo/resize.html mode, outside-in in
normal, fixed-height mode).

Closes #775

This makes it easier for alternative keymaps to hook into it.

Since v2 is the canonical CodeMirror at this point

So that I can put other doc-related stuff there without polluting the
root dir.

It's tiny anyway, no need to force people to include yet another file.

Much nicer than onKeyEvent

The complete.js script can now be dropped into any page

This normalizes the classes used by most of the modes
(reStructuredText and diff didn't fit in), and makes them all
reference a central style sheet (under theme/). It also adds a theme
option that can be set to change the theme, and defines a few
different themes (some based on old mode colors).