Add automated testing and snapshots with pdftest

[eKoopmans]

This PR:

• uses Karma to run Mocha/Chai tests in the browser
• adds unit tests for basic functionality and settings
• adds snapshot tests for:
  • blank PDF
  • lorem ipsum text (with default settings, a 1-inch margin, and "legacy" mode)
  • all visible HTML tags
  • CSS selectors
  • page-breaks (legacy mode, CSS mode, avoid-all mode, and specific-selectors moe)

Known bugs

• all-tags.html:
  • omitted due to bugs: <q> and <iframe>
  • styling errors: <col>
  • render errors: <details> (number instead of icon), <select> (no icon), <bdo> (doesn't reverse text)
  • not rendered at all: <meter>, <progress>, <embed>, <object>, <audio>, <video>
  • some questionable rendering choices for the <form> / <input> section
• css-selectors.html:
  • "unincluded ancestor": this is a known bug related to how html2pdf.js clones elements, should be fixed soon

Future todo

• set up CI for this with Saucelabs
  • test on PRs/pushes
  • add badge to readme
• more unit tests and snapshots

Other notes

• this was a very long work-in-progress (work began in April 2019)
• motivated the creation of pdftest

[eKoopmans]

Removed a code-fix for set({ pageSize }) that was discovered while building tests. Will make a separate PR for that fix.