consider a liquid macro to include svg icons as needed; support full library

[ellemenno]

currently all icons are included whether used or not; consider instead a liquid macro that pulls icons at build time, taking care of the html/svg/css.

also consider returning svg ids to the names found in the icon libraries themselves (maybe an alias to translate the programming-pages intentions back to library ids: interface → neuter)

e.g., instead of this:

<svg class="icon"><use xlink:href="#icon-folder-outline" /></svg>

something like this:

{% include icon.liquid icon=folder-o %}