resolves #70 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