remove requirement for providing a message

[st-h]

fixes #259

I figured it would be reasonable to keep the message property required when preventDuplicates is enabled as that feature relies on the message string.

Please let me know or feel free to modify the added test, if you know of any better way to assert a thrown error than that:

assert.throws((() => {
  service.add({ });
}).message, 'Assertion Failed: The flash message cannot be empty when preventDuplicates is enabled.');

[sbatson5]

Awesome! :shipit: