Add basic serve command

[tak1n]

Unfortunately I'm not that confident in rust yet so no tests for that.

Also do we need a test for just a basic command like this?
We are just using nickel therefore nickel should have a good test suite and done :D

This closes #50

[johannhof]

This should be in main.rs and not in cobalt.rs, as it's more a convenience functionality provided by our "frontend" and not part of the generator.

I'm good with no tests for this

[tak1n]

Done

[johannhof]

So we can make followup bugs for this, but there are two things that come to mind right now:

• serve isn't very useful as it is without watch, we should implement that
• the user should be able to specify the port to serve on

But yeah, let's make some followup issues for that

👍