Improve search support

Allow user to select between Lunr search (default) or [Algolia](https://github.com/algolia/jekyll-algolia).

Look into integrating search toggle into the masthead to avoid having to load a dedicated search page as per [Basically Basic](https://mmistakes.github.io/jekyll-theme-basically-basic-algolia-search/) and [Minimal Mistakes](https://mmistakes.github.io/minimal-mistakes/).