Fixes zsh-users/zsh-syntax-highlighting#146

by extending the already in place mechanism of checking for the first
histchar to check for the second as well

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.

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.

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

Leaking a variable that holds a path in CDPATH could easily end up appearing on your prompt like `~cdpath_dir/subdir`.

Parse sudo command line in order to highlight the options correctly

Comments added

Add support of CDPATH

Prepended an 'if' to remove error bell on osx systems.

Signed-off-by: Takeshi Banse <takebi@laafc.net>

Signed-off-by: Takeshi Banse <takebi@laafc.net>

Signed-off-by: Takeshi Banse <takebi@laafc.net>

Signed-off-by: Takeshi Banse <takebi@laafc.net>

* use a nicer variable name

* reuse $pos instead of using another variable
* remove declaration forgotten in last commit