Remove nightwatch in favour of puppeteer

[jalkoby]

Hey there.
I'd added the library to my project and it'd worked well, so I decided to make a little contribution as a favour. I see that you've been having a problem with tests, so I reworked them and replace with a more stable puppeteer. Only before the commit I noticed that there is a multi browser env, so the change will cut it and stick with Chrome only. Probably you will reject it and this is fine, it's your project. But nothing wrong is to make a shot at least

[sudodoki]

First of all, thanks for contribution!
Tests won't pass on travis - issue with clipboardy dependency, possibly can be solved by adding sudo apt install xsel to travis setup step
Tests working exclusively on Chrome are better than no tests at all, I guess, although main issue with this library was different behaviour across the browsers (like extra new lines in FF that I found after initially setting up saucelabs tests / etc) - so I would gladly merge this if you could make it work on travis

[jalkoby]

running xsel on travis ci is hard. I made a few changes, so currently I got 1 pass & 3 fails. Need more time to figure out how it should be configured