Add Katex support, Katex rending using https://github.com/Khan/KaTeX
Cover
```katex
x^1
```
to x^1
equal Katex expression
Katex extension for showdown
Add Katex support, Katex rending using https://github.com/Khan/KaTeX
Cover
```katex
x^1
```
to x^1
equal Katex expression
Katex extension for showdown