Move install script running to postinstall script

[karlhorky]

Run install script in postinstall, so that the only command required to install the config will be:

$ yarn add --dev @upleveled/eslint-config-upleveled

[karlhorky]

This doesn't work as I imagined, postinstall scripts in a dependency don't run in the project where the dependency is being installed (eg. they do not run on yarn add --dev)

[karlhorky]

Superseded by #165

(This can be done by publishing a create- starter kit package)