Consider using Snowpack plugins for Microsite's dev and build modes

[natemoo-re]

I will say as a matter of opinion, I think it would be a good thing for DX generally if the entire dev and build flows were packaged as Snowpack plugins, or each as a set of plugins if there's interest in picking and choosing certain behaviors, or if flexibility with ordering of build steps is needed. That would probably eliminate the need for Microsite to muck around with a user's custom config at all, and the dev and build flow would be a bit more clear. But I'm not sure how feasible this would be, at least without some significant rework.

Originally posted by @eyelidlessness in #92 (comment)