KaTeX incompatible with goldmark

[zhengzangw]

In current Hugo version, goldmark cannot correctly render '_' in math environment.

See Hugo Issue #6544

[zhengzangw]

By replace "_" with "_", KaTeX can be rendered correct. By setting mathmode=true in config, open this function.

With mathmode=true, there are two problem further

• One cannot use "" as markdown syntax. To type "", only "_" is needed
• Markdown Render Hook will be disabled.

Since these two problems are minor, we will not fix them for the moment.