[Feature request] Support markdown Highlight

[kehanlu]

Hi,

Thank you for developing this great plugin. I've been thinking about how the colorful concept feature could be expanded and have a suggestion. Would it be possible to extend this feature to include text highlighting? For example, users could highlight text using specific markers like this:

• ==&yellow Highlight==
• ==!red Highlight==

I think this can be done by filtering the <mark> tag.