[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
Add specialChars and specialCharPlaceholder options

Issue #649
Fix two bugs in f0610e60c3b220

Issue #649
Run mode over the whole line in long lines when in background-processing mode

Issue #1908
Fix completely botched IE11+ check
Export lineNo utility

Issue #1969
Make lineNo a method after all

Issue #1969
Add (undocumented) wholeLineUpdateBefore option

Issue #1972
Issue #1967
[placeholder addon] Don't hide placeholder text on focus

Most modern browsers now keep displaying the placeholder even if the
user has focus on the input and only hide it when the input has content.

This change is to make this addon consistent with this, so users won't
tell the difference with native inputs' placeholder behaviour.
Mark release 3.20
Bump version number post-3.20