sidebar toggle

[cehfisher]

feature request for a sidebar toggle option to show/hide the SG component list

[cehfisher]

@bradfrost

Added accessible button toggle element that expands/hides the sidebar with mouse click or keyboard navigation. Just tab to the button and press space bar or enter to toggle.

Added visuallyhidden selector to hide the toggle button name from visual users, but will be read for screen readers (so those users know what they are toggling). This selector can be used for other similar elements or to give more context to links (for example: http://a11y-style-guide.com/style-guide/section-general.html#kssref-general-read-more).

For the actual style of the toggle, I created something simple to go with the rest of your style guide ascetic, but update what you need to. Also, I did some quick tests on Chrome, Safari, and FireFox (latest versions), but might need some deeper cross-browser testing.

See #21 for code. Hope this helps get you started on the sidebar toggle :)