mutable head extension (#17)

* inital draft of mutable head extension

* a couple formatting tweaks; added an example rewritten inventory

* change 'Extension ID' to 'Extension Name'

* add placeholder extension number

* updated based on pr feedback

* update line formatting

* fixed a few minor typos

* add extension version

* remove index changes

* PR fixes

* Update commit definition

Co-authored-by: Peter Winckles <peter.winckles@wisc.edu>