Changing the default launch message

[Stefanuk12]

The problem

You cannot change the launch message easily as it's hardcoded as seen here. You currently have to disable all logging in order to achieve this, but you might want to keep the rest of the logging, and only change the launch message.

A possible solution

An optional parameter could be passed within the launch function which is passed to the logging macro instead of the hardcoded default. An example usage could be like this:

rocket.launch("Server started")

This may break existing codebases so to minimise this, a function called launch_with_message could be used instead.

[SergioBenitez]

I don't think we need any explicit APIs to support this. When we get to #21, we should make sure that removing this message is pretty trivial, however. Adding your own message is already trivial via a launch fairing.