Followup to 0d1bfbcbfa.

Reported-by: Antony Lee

Fixes zsh-users/zsh-syntax-highlighting#166.
Fixes zsh-users/zsh-syntax-highlighting#194.
Builds upon the issue #207 work.

Update expectations on the subshell test (which triggered this bug, since it has
a passing test point following a 'todo' test point).

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 issue #195.

* 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>