Add offline search to preview sites

[theletterf]

We often use preview sites as internal documentation sites, but they lack search capabilities.

I propose adding offline search to preview sites (and preview sites only) so as to grant some degree of findability in the content.

• Offline search could be toggled using a docset.yml setting (offlinesearch: true).
• We could use something like http://elasticlunr.com/
• The offline search index would be built when the preview site is built, if the setting is enabled.

[theletterf]

Might not be necessary after all if we end up using ES. :)