test failure would not be reflected by the exit code of 'make'.

Setting a shell parameter in the left-hand side of a pipe is not visible to
commands after the pipe, because the left-hand side forks.  (That's true both
in 'sh' used by 'make' and in 'zsh' that runs tests/test-highlighting.zsh, at
least on my system.)  Therefore, move the colorizing hook to where it doesn't
interfere with setting the $something_failed (in tests/test-highlighting.zsh)
and $result (in Makefile) parameters.

Color is only added when "make test"'s stdout is a tty.

Fixes zsh-users/zsh-syntax-highlighting#217.

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

That allows tests to be completely independent of each other, so tests that
change global state — such as modify environment variables, define functions or
aliases, or hash commands — will not affect other tests, without needing an
explicit cleanup step.

This enables testing path-tilde-home with and without $HOME set, which is part
of issue #216.

While at it, convert the test harness to TAP.  This fixes issue #180 by adding
support for "not ok 42 # TODO" output.

This commit assumes that 'grep' supports POSIX-compliant -q and -v flags.

Patch-by: Matthew Martin <phy1729@gmail.com>

In region_highlight, a spec of the form 'i j foo' with i >= j should have no effect.
Before this commit, however, the {$i..$j} range would happily expand to (5 4 3)
if i > j were the case (e.g., i=5 and j=3).

This breaks vanilla-newline.zsh; the next commit will fix that.

This way, each test can set custom styles to specific highlight contexts
(to differentiate those contexts) without affecting later tests.

Otherwise, *.rej patch files would be run by zsh, with unpredictable results.

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>