Improvements of corner cases: default printing style, icon on dark backgrounds.

Improved support for dark and light color-schemes, with new dark-mode github css and optional automatic switching between light and dark highlight.js code schemes.

markdown-viewer now supports dark and light modes out-of-the box, and can optionally render LaTeX!

Fixes the code-coloring dropdown issue, and releases old fixes such as visible overflow of long markdown menus.

Table of Contents menu.
Drop-down on menu to change style.
Button on menu to export HTML.
(Above features contributed by Cimbali. Thank you!)
Emoji support.
(Above feature contributed by BlueHeaven1969. Thank you!)

No longer works in Chrome; it does not follow web-ext spec for storage.get().

- Obtains its permissions up front; no need for browser action icon to request permissions.
- Expanded the project README with oft-requested information; linked to it from the options page.
- Now works in Chrome browser as well.

- On page refresh, remembers and scrolls to current position in the document.
- Adds anchors for header elements.
- Supports remote URLs using optional permissions, requested via browser action button.
- Supports user-defined CSS for Markdown in the options page.

Elaborated description.