Add concise method support for functions name 'get' and 'set'
This patch modifies getter and setter checks so that if the esnext flag is
enabled jshint will not treat functions named 'get' and 'set' as
getters/setters (respectively) *unless* there is a property name specified
after it. For example:

    var ins = [];
    var outs = [];
    var SimpleMap = {
      get(key) {
        return outs[ins.indexOf(key)] || null;
      },
      set(key, value) {
        ins.push(key);
        outs.push(value);
      },
      has(key) {
        return ins.indexOf(key) >= 0;
      },
      delete(key) {
        var idx = this.indexOf(key);
        if (idx >= 0) {
          ins.splice(idx, 1);
          outs.splice(idx, 1);
        }
      }
    };

If one were to run this code through jshint with esnext enabled, it would
complain about the `get` and `set` methods, even though this is perfectly valid
es6 code. This patch fixes this, but still complains when esnext is not
enabled.