Add support for the alias function

[Jeff Rescignano]

TinyURL supports adding aliases to your URL (more on this here). For this issue, a function will need to be created that will allow users to add an alias when creating a short URL. The function should also return an error if that alias is already in use. For information on how the TinyURL API works with aliases, I'd recommend reading this question on StackOverflow. Please reply if you'd like to be assigned to this issue.