Correct highlighting of backslash escapes within "" strings: highlight only
the four specific escape sequences defined there.
Fixeszsh-users/zsh-syntax-highlighting#196.
* 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
