Closes #497

Closes #468.

The fix for issue #9 was accidentally broken by the penultimate commit.

Note that we don't care about aliases to ZSH_HIGHLIGHT_TOKENS_COMMANDSEPARATOR
entries, since the code deals with aliases at command position.

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

Fixes zsh-users/zsh-syntax-highlighting#189.

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>