Add a port number as an optional config value

[chebykin]

This will solve #65 if you specify the same port number in eth-gas-reporter and test-environment configs. When you ignore a port number value in your config, it will use a default undefined just like before.

[frangio]

Thanks for the suggestion @chebykin! This is an interesting way to achieve support for gas reporting. As a side effect, though, degining a port number will make parallel tests (with Jest or AVA) stop working. So I'm not sure the solution is acceptable for us.