Closes #2960

Issue #2952

Closes #2955

Issue #2928
Issue #2958

The gfm mode is for the GitHub flavour.

Using the collapseIdentical option.

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