Allow use of the 'const' keyword in place of 'var'

This isn't standard and shouldn't be used in portable code, but it's
required in some places for XULRunner extension development where I'd
still like to use JSLint.  It's also supported by V8 in Chrome and
Node.JS.  So allow it with the "constvar" option.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>