Fixes #494

Closes #379.

Fixes #480.

Closes #277.

The fallback style name uses '_' in anticipation of a future auto-fallback
feature keyed on style names.

'arg0' was previously known as 'commandtypefromthefuture'.

The new text is from the preceding two commits, 22123ab622 + ded1c8789b.

Fixes a broken link.

Fixes zsh-users/zsh-syntax-highlighting#187.
Adds a test for zsh-users/zsh-syntax-highlighting#204.

Correct highlighting of backslash escapes within "" strings: highlight only
the four specific escape sequences defined there.

Fixes zsh-users/zsh-syntax-highlighting#196.

Fixes zsh-users/zsh-syntax-highlighting#23.

This does not yet highlight backslash escapes within them.

Fixes zsh-users/zsh-syntax-highlighting#126.

* Don't override user defined styles
* Better modularisation of highlighters
* Allow to define which highlighters are activated
* Allow to define the order in which they are defined
* Minor performance optimizations
* Fixed some variables leak
* Improve documentation
* Brackets highlighter: use ZSH_HIGHLIGHT_STYLES instead of a specific array