Port site to metalsmith

[edrex]

Value proposition:

• Configure your own pipeline by stringing together a few plugins
• Lots of available plugins, very little functionality in core.
• Plugin API is stupid simple: you get passed a map of file metadata (with data stream in content key), which you can mutate in place.

This port will require a few smaller issues

• Load _data.json files via plugin, to allow in-situ migration of public dir
• Port template handlers as needed