Add positional parameter instead of hard-coded port

(In the same fashion as [Echidna PR #4](https://github.com/w3c/echidna/pull/4).) Hard-coded config details make it hard for anyone to set up and test Specberus. The port number can now be given as a positional parameter (see [the new documentation written about this](https://github.com/w3c/specberus/blob/20d45020cfd84508f60cd944749615fc145c9568/README.md#syntax-and-command-line-parameters)).