Finds highlighters dir a little better
add separate path_prefix and path_approx hilights
Fix #121: sudo syntax highlighting incorrectly

Parse sudo command line in order to highlight the options correctly
a more simple and universal way (for zsh projects) to find highlight dir path
Do not leak a variable: cdpath_dir.

Leaking a variable that holds a path in CDPATH could easily end up appearing on your prompt like `~cdpath_dir/subdir`.
do not remove quotes when checking assignments

Zsh does not allow the variable name or the equals sign to be quoted or
escaped.  The previous code incorrectly highlighted the following
examples as assignments:

    $ 'x=y'
    zsh: command not found: x=y
    $ x\=y
    zsh: command not found: x=y
    $ "x"=y
    zsh: command not found: x=y
    $ \x=y
    zsh: command not found: x=y
highlight array assignments of the form x[y]=...

This code is more lenient than bash.  Examples:

    $ x[y[]=
    zsh: no matches found: x[y[]=
    $ x[][]=
    zsh: no matches found: x[][]=

The proper solution is to look inside the [...] and make sure that all
unescaped/unquoted square brackes are matched, but that is a heck of
a lot more complicated than this simple 8-character patch.
protect alias expansion from interpreting aliases as options

If one defines aliases like `++` the alias builtin tries to interprete these
as options so they have to be protected like this

    alias -- ++=true

The same goes for a call to `alias` in order to expand the alias again.
#110: Now using :A modifier, bump zsh version requirement
add separate path_prefix and path_approx hilights
Customize highlighters.