#9 add support for "exported" directive for JSHint

[ChrisRaven]

Add support for "exported" directive, so when functions or variables are defined (but not used) in one file, but used in another, JSHint doesn't mark them as unused, e.g.:

/* exported abc, msg, tpl */
var abc;
function msg() {}
function tpl() {}

The above code should work without any warnings.