Add option to change the default start script

[adamgyulavari]

Added a new configurable option startScript, that's loaded in the utils with the default value start and used in the yarnStart. Mainly did this because on the (Macbook's) Touch bar there's only a start button. I could've made that changeable, but maybe there are more features a dev is using around the default start behaviour, so I think it's better to change that.