Merging Butane configs

Created originally in Butane by @bgilbert: https://github.com/coreos/butane/issues/118

It's convenient for users to write multiple Butane fragments and then merge them together into one config. This is awkward right now: users must separately transpile each fragment and then use Ignition's merge directive to merge them at runtime. Or, they can transpile each Butane fragment and use a wrapper script to build a top-level Butane config that inlines the Ignition fragments using merge directives.

Provide a mechanism to include Butane fragments into another Butane config and produce a unified Ignition config as output. This might be done by extending merge to allow referencing local Butane configs, transpiling each piece, and then performing the Ignition config merge at transpile time.

Start by reviewing how Ignition currently handles the merge directive and how Butane fragments are transpiled. Define the transpile-time merge behavior and its supported local-config references before identifying implementation points and tests; done means multiple Butane fragments produce one unified Ignition config.