Add support for Jekyll plugins via the gem mechanism

[ochrons]

Currently microsites supports Jekyll plugins only by copying the plugin code into a local directory. It would be preferred to support the regular "gem" plugin system used by Jekyll. This would make maintenance and versioning of the used plugins much more straightforward.

With the current model, the plugin source must be included in the documentation source (and in the repository), which is not that nice.

See for example https://github.com/scalafiddle/scalafiddle-core/tree/master/integrations/jekyll