Notably, katex goes from 0.6.0 to 0.16.0 and (for testing purposes
only) markdown-it from 6.0.0 to 13.0.0.  Several tests had to be
updated as katex’s HTML rendering of many math constructs has changed,
but no code changes are required.

Fixes issues waylonflinn#18, waylonflinn#19, waylonflinn#21, waylonflinn#27. Partially addresses waylonflinn#16.
Test cases added.

By default, invoke KaTeX in ‘throwOnError:false‘ mode, so that it will
produce _something_ from any input, and in particular, something which
is guaranteed not to allow any live HTML tags through from the input.
(Unless you are using ‘trust:true‘, in which case you presumably know
what you are doing and why.)

throwOnError:true mode can be re-enabled with plugin options, but then
this plugin will **not** catch any exceptions that are thrown, leaving
recovery up to a higher level.

Since this is a significant behavior change, but one that is more
likely to *fix* bugs in dependent programs than to introce them,
the version number is now 2.1.0.

Fixes waylonflinn#26, waylonflinn#30, and waylonflinn#7.  I would like to thank @khrykin, poster of waylonflinn#7,
for the idea that maybe the exception handlers inside this plugin
should just be completely removed.