Support Fetch for APIs

[adgad]

For APIs, it would be good to not have to install puppeteer and Chromium to run smoke tests.

Proposal

Make Puppeteer and Webdriver.io installable "drivers" (need a better word). The default would be fetch (+ maybe JSDom).

An app that wants to test puppeteer would then have both n-test and n-test-puppeteer installed.

n-test-puppeteer could be either required in the test file itself? Or maybe a .n-test.js in the root, with global options (including additional custom checks etc).

Something like that.