Update save_kable.R to include MathJax

[zhangchuck]

Fix for #578?

This commit makes the HTML table save_kable_html() function mirror this:

kableExtra/R/print.R

Lines 12 to 15 in 4c93f1a

	htmltools::HTML(
	as.character(x),
	'<script type="text/x-mathjax-config">MathJax.Hub.Config({tex2jax: {inlineMath: [["$","$"]]}})</script><script async src="https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>'
	)

Otherwise, MathJax is not included in the HTML file (which affects the image outputs).

[zhangchuck]

This work if you invoke with self_contained=F:

save_kable('test.png', self_contained = F, delay=1)