Use filename as default contentIndex.json title

[njncalub]

Hi, is it possible to add a flag that can use the filename as the default title on contentIndex.json?

Reasoning:

Some markdown pages do not have frontmatter data, and because of this, the Backlinks section will have empty divs, and the interactive graph will show the relative path instead.

Thanks

[njncalub]

I created a sample commit here: njncalub@8cd0248

My Go is a little rusty, so I'm unsure if that's up to par.

Thanks!