You forgot to mention that nightwatch needs to be installed globally in order to run the nightwatch command.
Running the nightwatch command results in "Error: Unable to access jarfile ./node_modules/selenium-standalone/.selenium/selenium-server/2.53.0-server.jar"
nightwatch command
.nightwatch
command results in "Error: Unable to access jarfile ./node_modules/selenium-standalone/.selenium/selenium-server/2.53.0-server.jar"