feat: add `-w` to automatically generate table of content when target markdown file changes

[pangrr]

I feel this feature can save me time from enter command multiple times to update table of content while I am adjusting a markdown file. I am doing some literate programming in markdown files, so this is a common situation for me. I guess this feature helps for heavy markdown editors like me.

Plus it's natural for a tool meant to automatically generate table of content from a content to be more automatic.

-w should be able to work with other options.