Closes #4105

- Images now come with tons of information to style them. This mainly
  enables to style alt-texts and potentially make them look like links.
  While still differentialting between links and images.

Closes #4082

Closes #4085

It is, per https://golang.org/ref/spec#Predeclared_identifiers

Closes #4064.

Closes #3888
Closes #4063.

Closes #3796

Closes #3900

Closes #3926
Closes #4026

Add gutters properties (grid-gap, grid-column-gap and grid-row-gap):
https://www.w3.org/TR/css-grid-1/#gutters

Take advantage to add some missing keywors:
* grid and inline-grid: https://www.w3.org/TR/css-grid-1/#grid-containers
* dense: https://www.w3.org/TR/css-grid-1/#grid-auto-flow-property

It's been a while since the list of properties has been updated:
https://drafts.csswg.org/css-grid/#property-index

1. Mention multiplex as a dependency
2. Add ERB as a MIME-type

Closes #4020

Closes #4012

Closes #4009

Issue #4013

Closes #4013

Closes #3994

Issue #4001

Use it to make the C++ mode not indent template statements

Closes #4001

according to Clojure's changelog: https://github.com/clojure/clojure/blob/master/changes.md

Closes #3981

Issue #3973

@ as __matmul__ operator only on Python 3,

Add tests for Python mode for this use case.

Closes #3957

`u` prefix are valid (again) since 3.3, and `f` prefix are now allowed
starting at 3.6 (which is not released yet though).

There were two issues:

1. Github flavored markdown calls Objective C `objective-c`
2. The gfm.js mode didn't include `-` in its fenced code block regex

Additionally, test/index.html did not import the mode/meta.js file,
which prevented mode lookup by name from working in unit tests.

Issue #3932