tests/generate.zsh already does this.

The overall per-highlighter duration should be less prone to random
noise than the multitude of per-test-file figures.

Currently, without zsh/zle loaded, the tests silently fall back to the
5.2-and-earlier codepath; see:
.
    https://github.com/zsh-users/zsh-syntax-highlighting/pull/356#issuecomment-243651251

No functional change, except that error messages are now more readable.

In particular, tests were relying on the harness to provide a sandbox working
directory, but the perf harness wasn't providing that.

* 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>