PDF embedding

[rufuspollock]

![[xyz.pdf]] turns into a pdf embed that works in major browsers.

Acceptance

• [x] ![[xyz.pdf]] turns into a pdf embed ...

Tasks

• [x] Research how you can display a pdf in html in major browser (maybe there are no longer good ways) and recommend
  • [x] Identify fallbacks - maybe we just display a pdf link instead of embed
• [x] implement

[abhishekgahlot]

PDF embedding is done via embed in the following PR: https://github.com/flowershow/remark-wiki-link-plus/pull/6