Fixes #585.

Fixes #333.

Followup to b2ac98b981.

Make the test pass by setting explicitly the global state (existing $HOME) it
depends on.  Test another value of the global state (a non-existing $HOME).

All these files should be sourced, not executed; and Debian's lintian complains:

W: zsh-syntax-highlighting: script-not-executable usr/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh

Signed-off-by: Takeshi Banse <takebi@laafc.net>

* 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

Feel free to add more test data !

Signed-off-by: Julien Nicoulaud <julien.nicoulaud@gmail.com>