Fixes https://github.com/codemirror/CodeMirror/issues/3215

Issue #3214

This changes will make the optional space character after ATX style
headers be included within the `cm-formatting` element.

https://msdn.microsoft.com/en-us/library/ms189822.aspx

Closes #3172

Closes #3207

https://github.com/codemirror/CodeMirror/issues/3205

Issue #3204

And change handlebars demo to use it.

It was never standardized and is no longer a reserved word: http://publications.gbdirect.co.uk/c_book/chapter2/keywords_and_identifiers.html

Currently and/or/not in most themes look like normal text because cm-operator doesn't usually get special styling. This patch highlights them as keywords to make them stand out better.

 - Add string recognition
 - Include (most probably) all built-in template tags
 - Return null as fallback
 - Operator highlighting
 - Change demo theme to `mdn-like` which sports richer highlighting.
 - Highlight comments
 - Define `blockCommentStart` and `blockCommentEnd`

It was removed in 957771b2

So that dragging and then coming back to origin point does not
clear the selection.

Issue #3151

Clean up duplicated logic

Issue #3189

Closes #3188

Issue #3186

For example:

v1 = $v2
  |new line

body
  color $c
    |new line

Issue #3173