Toggleable Sidebar

[rrcobb]

Issue: #32
Ref: #33

Adds a button to hide / show the sidebar.

• Button is unobtrusive, it's just ☰ (U+2630)with a little padding
• For screens < 1080px, sidebar defaults to closed
• The js is a little inline script tag, should be easy to find / modify if anyone has a bright idea
• The CSS is a little cute; it uses a negative transform to move the sidebar away so that it can have a transition.

This PR is stacked on top of the fixed-width sidebar PR. Compare just the diff that adds the JS

[gvwilson]

I like the toggleable sidebar, but other people have suggested stacking the ToC on top of the content for mobile devices; I've also made a few changes to your changes to mccole.css (thanks for those) that may introduce some conflicts. I'll take another look at this one tomorrow - thank you for submitting it.

[gvwilson]

I've pushed changes that move the navigation to the top of the page for narrow devices - please check it out. If anyone has more thoughts on toggling, please let me know.