Running both starting scripts in a single terminal

[Rafajrg21]

When I forked the project and started my developing process I thought about how it would be easier to run both starting commands (yarn dev and hugo server) in a single terminal.

My proposal is to add the concurrently package to the Dev dependencies and use it from a single start script. Personally I think that's a better developer experience than having two terminal tabs/windows open.

[baltpeter]

Thanks for the suggestion. So far, we haven't done that as both Hugo and Webpack produce a fair amount of (sometimes relevant) terminal output and this approach would mix them, making it harder to read.

And with the changes in #701, this won't be necessary anymore anyway, as we are then building the JS through Hugo as well.