[htmlmixed mode] Fix bug in intra-token-closetag detection

Issue #1943
[show-hint addon] Fix event delegation for UL/LI

It's possible to provide custom markup for hint elements that are LI
tags and that breaks event delegation logic that expects UL to have
text-only LI nodes. As a result if you have elements inside LI and
click/dblclick on those, nothing happens and selected hint is not
applied. The fix improves event delegation in the way to find nearest
parent element to event target that is a hint element at the same time.
[show-hint addon] Style fixes

Issue #1947
[show-hint addon] Allow a specific option to be selected by default
[real-world uses] Update link to Handcraft
[markdown] Emit 'header1'..'header6' styles in addition to 'header'.
[ambiance theme] Text selection uses default background color

`cm-s-ambiance` and `CodeMirror-focused` are classes to the same
element, so search for `.cm-s-ambiance .CodeMirror-focused` will fail.
[docs] Remove underlines for headers
[show-hint addon] Fix bug introduced by merging 1013ebb17f89375e
[closetag addon] Fix bug in code that updated the autoCloseTags option
[javascript] Include ES6 for..of construct.
[javascript mode] More ES6 features (modules, destructuring, generators, class)
[javascript mode] Add ES6 fat-arrow support
[javascript mode] Add support for ES6-style comprehension
[javascript mode] Add support for ES6 quasiquotes
[javascript mode] Use 'extends' instead of parenthesized syntax for inheritance
[javascript mode] Fix indentation of 'else' lines

Issue #1960
[fullscreen addon] Set with to auto rather than ''
[dialog addon] Make dialogDiv accept a detached DOM node in addition to a template string
[foldgutter addon] Add options to customize change/update time spans