Extra buttons to support

[choldgraf]

from @TomDonoghue in the previous version of this repo:

Possible small feature addition - happy to make a PR if this seems interesting:

As I was using this for a course website, I first wanted to remove the 'Interact' button, and then add a 'Download' button (to directly grab the notebook from the site).

So, I added Button options are at the top of the config, for a group of 'buttons' I might use at the top of the rendered notebooks (and associated code to make them work through the repo):
COGS18/COGS18.github.io:_config.yml@master

^I'm not sure what set of buttons one might want to support in the base version, but I think at least a toggle for having the 'Interact' button might be nice (and implies one can easily extent for others), and maybe downloads too (I also have a pdf button that is probably much more idiosyncratic).

Sanity check - features comments / things should go here, and not the demo notebook? Also, my course textbook is private right now but you two (Sam & Chris) should have invites, if you want to have a look.

[choldgraf]

this is now implemented in #38