Feature/copy html to clipboard

[klepas]

Changes:

• adds clipboard.js v1.5.15 (minified, w/ version info)
• implements clipboard.js via ‘Copy HTML to clipboard’ buttons for each HTML code snippet block
• adds github icon to the github source anchor <cite> (added to spritesheet, along with PNG fallback, both optimised)
• adds a fix for the skip-to link rendered example space, padding it, and adding text via CSS to inform the user it holds a visually hidden example

Screenshot:

Testing site: http://dg-html-to-clipboard.apps.staging.digital.gov.au/

[klepas]

This also fixes https://github.com/AusDTO/dto-design-guide/issues/43#issuecomment-264750758