Make port configurable in projectConfig

[luke-m]

I sometimes need to run two projects locally at the same time. Because biotope-build's serve process uses port 9000 by default, that's not possible without changing the used port. Currently it's possible to change it in biotope-build's config.js file. However, because that's a change in the node_modules folder, it gets lost upon every yarn install execution.
My suggestion: make the port configurable in the projectConfig.js file like a lot of other features are.