* Add optional KaTeX support to the tiddlywiki/markdown plugin.
Uses the remarkable-katex plugin 1.1.8 by Brad Howes to enable KaTeX support if
the tiddlywiki/katex plugin is installed. Fixes#2984.
TESTED:
Created a test wiki with:
```
$ node tiddlywiki.js test --init markdowndemo
$ node tiddlywiki.js test --listen
```
* Verified markdown support works without the tiddlywiki/katex plugin enabled.
* Verified markdown support works with the tiddlywiki/katex plugin enabled.
* Verified KaTeX (both inline and blocks) work as expected when the
tiddlywiki/katex plugin is enabled.
* Mention remarkable-katex plugin usage in the readme Tiddler.
* Include the remarkable-katex license as a tiddler.
* Include the Remarkable license.
* Include unminified original source of remarkable-katex 1.1.8.
