Fix for #2: Local timeouts for each flash message

Sometimes for fine control over UX, we need to be able to control timeouts for each Flash message instance(s).

e.g.: `Flash.set('top', 'Awesome message disappearing in 3 secs', 3000);`


We needed this feature on an app I was working on. Personally tested this and it works great!

Also updated the documentation to match the new feature.