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

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

Related to issue #205.

Notably, the 'single-hyphen-option' and 'assign' contexts both default to the
'none' style, so before this patch, the tests would not have detected a failure
to apply those two contexts.

For other contexts, using $unused_highlight only helps detect the case where
the right highlight style (e.g., 'fg=yellow') is used as a result of applying
the wrong context — which should be a theoretical failure mode.

This is part of zsh-users/zsh-syntax-highlighting#198.

Fixes zsh-users/zsh-syntax-highlighting#181.

* 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>