Shrinkwrap marked dep. Galaldirith/marked#mathjax

This fork of marked, based on a previous fork by
summivox/marked#mathjax can parse inline and display equations in a
manner familiar to LaTeX users.
Update LATEX.md with relaxed delimiter syntax
Remove redundant methods in mathjax-helper
Add option to enable LaTeX rendering by default
Add instructions enable LaTeX rendering by default
Merge remote-tracking branch 'upstream/markdown-preview-plus'
Update merged upstream changes mp -> mpp
Add troubleshoot for forked marked dependency