so other potential emacs-type modules can use

Fixes #2470

Issue #2263

As long as CodeMirror.normalizeKeyMap is called on them before they are
used. This interface is a little weird, but backwards-compatible.

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

Issue #2507

Issue #2507

pressing Ctrl-Space once starts the selection extension mode,
pressing it again cancels that

Since it's what you want most of the time, and easy to forget.

Issue #778

Closes #2142

Make them support prefixes.

Stop suppressing key events for nofallthrough keymaps.

Issue #1558

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

Make character bindings work for maps that define a nofallthrough.

Issue #274