Update to Octicons 5.0.1

[coliff]

REF: https://octicons.github.com/

[thomaspark]

This will require some rework since Octicons 5 has dropped icon fonts in favor of SVG: https://github.com/blog/2112-delivering-octicons-with-svg

Several of the copy options like unicode and hex are no longer relevant, and markup will change from <i class="mega-octicon octicon-alert"></i> to something like <svg version="1.1" viewBox="0 0 16 16" class="octicon octicon-alert" aria-hidden="true"><use xlink:href="sprite.octicons.svg#alert" /></svg>.