Add submenu headers

* Putting a # in front of a list-item in a submenu makes it a submenu header
* Closes issue #21
Re-enable in-page anchors

* Fixes issue #19
* Shows a paragraph symbol when hovering over an in-page anchor (=heading)
* Adjust link when clicking on in-page anchor
Bump version to 0.5.6
fixed typo in config.useSideNav and updated side nav heading links