Add ability to run external asset pipelines from Orchid

[cjbrooks12]

A common use-case would be to run Node-based build pipelines for frontend assets. The current way to add assets from these pipelines to an Orchid site is to simply run them independently, but have the Node pipeline run before Orchid, so that Orchid can pick up the outputs and use them.

There should be a way to have Orchid run these pipelines as part of its own build process. I'm thinking including something like a PublicationPipeline to the AssetsGenerator, which can be configured via config.yml. There could be pipeline stages for specific types of runners (gradle, NPM, yarn, etc), and it would be nice if they could "emit" output assets which get attached to pages automatically, so that the user doesn't need to manually set the output files, and so it would still work with dynamic (hashed) filenames.

Middleman's external pipelines seem to pretty much be the same idea that I'm thinking of here, for a better understanding of what I'm thinking of for this.