Issue #4864

This is opt-in, the plugin needs to produce message_html instead of
message to be rendered as HTML.

This should help developers building this functionality out that may not
realize they need to source the linting library seperately.

Issue #3059

Closes #4855

Because the borders don't make much visual sense. And they cause dual
scroll bar issue when the editor takes 100% of page height.

You ca reproduce the issue here: http://mdp.tylingsoft.com/ Press
<kbd>CMD-,</kbd> to bring up the preferences panel and change the
theme to midnight.

[sublime keymap] "Add to selection" bindings (Ctrl-Alt-Up/Down on Windows, Cmd-Shift-Up/Down on Mac)

Also correct links to specification.

So that the addon doesn't try to put a single DOM node in two places

Closes #4848

Modern browsers handle updates to a textarea's value fast enough
to not do this anymore.

Issue #2703

Closes #4840

Issue #4839

Issue #4840

Closes #4839

Closes #4835

This way, only one char's left and right edges are used, instead of its
(logical) neighbour's right edges.

In bidi content, this avoids trying to scroll to two logically close,
but visually far apart positions in some situations.