Allow for disabling the right hand side contents

I was trying to emphasize a piece of the documentation via #'s and this renders a contents section which is not useful to the user. Being able to toggle this would be helpful.

[jsonmaur]

I agree it should be configurable on a per page basis. But I also think the TOC should only show h1's or maybe h2's as well (# or ##).

[jsonmaur]

Fixed via #140 and #141.

It will no longer add TOC items for headers higher than h2 (###, ####, etc.) Can also completely disable page TOC with this front matter:

---
table_of_contents:
  page: false
---