(So that it plays nice with loadmode)

Also update the mode loading demo to make use of the info in that
file, allowing filenames and mimes to be entered.

Issue #2828

Added file extensions to the meta info. Added helper functions
getExtFromMimeType,  getMimeTypeFromExt, getFilenameAndExt from a file
name, and detectCodeMirrorMode from a mime type

Move keybuffer clearing code out of the matching code

Since the mouseover will necessarily happen at the very edge of the node, and thus
give ambiguous coordinates.

Closes #2821

Closes #2822

Issue #2819

Issue #2807
Issue #2814

This might break sites that depend on async linters getting an editor instance
as first arg, or sync linters getting the .options property of the options passed
as second argument. The args passed are now more or less consistent.

Ugh.

Closes #2816

Closes #2813

after header immediately followed by a code block, GFM formatting is
broken (SHA, #issue, URLs).

overlay.codeBlock gets it wrong (false inside fenced block, true after).
Not yet sure why.

Added hawkee.com

Throwing exceptions is a really bad way to deal with unexpected
syntax in a highlighting mode.

Closes #2805

Closes #2810

To make it easy for client code to color completions without clashing
with the selected styling.