Add Table of Contents support

[parabolapersuader]

Add Table of Contents support, say configurable as with the light/dark/auto styles. After all what's the point in having a Markdown browser if one can't use the structure offered by Markdown from the beginning?

You can try Markor, the Android text editor, as it handles the TOC in a very elegant manner, without adding the mess of having [TOC] tags, say like Joplin.