Choose a random port instead of always using 3000 by default

[e-n-f]

So that it is easy to have multiple instances of mbview running at the same time without having to set the port manually

[andrewharvey]

I'd probably prefer it try 3000 by default and then if it's not available use another random port. This seems to be what most other tools do.

Though the only advantage I can think of for mostly using the same port is it's more familiar to the user and it'll pick up the localstorage cache more often than not.

[e-n-f]

@andrewharvey I agree. I'll see if I can figure out how to do that.