feat(shields-svg): Adds shields.io SVG support

* Adds SVG support with {extensions: 'ext'} key within sendStatusBadge
* Adds a fourth argument to badgePath for extension
* Adds SVG files to src/img/status/shields.io
* Adds unknown for shields.io theme
feat(shields): Adds retina support for shields.io

* Adds @2x files for every status update.
fix(lint): Fixes lint and a typo.
fix(typo): Fixes a typo from options to opts.
Merge pull request #121 from durango/feat/shields-svg

feat(shields-svg): Adds shields.io SVG support