Added globals support to JSHint check.

Currently Zed only allows to override options passed to JSHint check command.

Now it is possible to also pass custom globals if project requires them.

For example in ./zedconfig.json you can put

```json
{
    "modes": {
        "javascript": {
            "commands": {
                "Tools:Check": {
                    "options": {
                        "globals": {
                            "READONLY_GLOBAL": false,
                            "WRITABLE_GLOBAL": true
                        }
                    }
                }
            }
        }
    }
}
```

As discussed in https://groups.google.com/forum/#!topic/zed-user/Xe4AdaO7PgE