Check for selenium-server.jar and chromedriver binary before downloading

[ellenmobify]

Summary

When installing nightwatch-commands, a copy of selenium-server.jar and a chromedriver binary are downloaded as well. If these files already exist, perhaps we don't need to redownload every time.

Expected behaviour

To save time and bandwidth, don't download selenium-server and chromedriver binary if they already exist locally.

Actual behaviour

Download every time.

Steps to reproduce

1. In package.json, use a different version of nightwatch-commands
2. npm install
3. selenium-server and chromedriver are downloaded even though a local copy of the same version already exists

[brettjonesdev]

You might check out selenium-download as it handles this exact scenario. May be worth using in this project instead of reinventing the wheel.