More natural zooming on block level elements

Switching a `display: block` element to `display: inline-block` allows calculating the bounds based on the contents of the div rather than the entire container (which is often `width: 100%`).

This provides a much more natural zoom, especially for paragraphs and code examples.

`./*.md` didn't work `*.md` does work

Fixes #1549, now UA match is limiting out MS Edge and Chrome detection is improved. Also improved UserAgent access by storing it in to a local variable, it was accessed so many times in different places.

readme tweaks

Table of contents added in README.md

Make PDF export section less OS X specific.