add copy to clipboard button to source blocks

add copy to clipboard button to source blocks

* add subsetted SVG icon sprite for octicons
* add copy to clipboard button to source blocks (visible on hover)
* replace pseudo-element for source language with a toolbox (managed by JavaScript)
* autodetect literal block with command and promote to a console source block
* intelligently extract commands from console source block, flatten them, and join them in a chain
* prevent command prompt(s) in console source block from being selected
* darken color for annotation on source block
* configure svgo to preserve desc element
* configure svgo to preserve ID prefixes for icons