Extract selenium install logic to a specific module

[Sergeon]

Currently, the whole nightwatch.conf.js is set as a postinstall hook in package.json. But the only reason to do that -as far as I understand- is to ensure that selenium gets downloaded. This is pretty misleading since people just copypasting the configuration object from nightwatch.conf.js to their own config file as a template would not get the tests to run, since selenium would not be downloaded.

Also, this way is just more clear what the postinstall hook is about, and concerns are better separated between the nightwatch configuration and the selenium install.