It's really annoying to have partial paths appear the same as paths to
complete files. However, it is still nice to be able to highlight them.
Full paths are now bold while partial paths use plain underline.
* 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