Layering Auspice config files

[genehack]

Context

Slack context

Description

Sort of regarding the auspice config stuff, it would be cool if we supported config merging or inheritance someday. For instance, if we had reasonable default auspice config we could pass to augur export along with a minimal custom config that overrides or sets just a couple of build-specific options (e.g., title, etc.) that could save a lot of copy/paste (or failing to copy/paste to all the possible copies when a new color-by is added) (from @huddlej in above Slack thread)

[jameshadfield]

Largely a duplicate of #298

[genehack]

sigh and I searched for a previous issue, dammit, but not in the AUGUR repo... >_<

[tsibley]

GitHub's org-wide search is nice for finding stuff precisely because of this problem. (Personally, I've defaulted to always using it, even if I'm pretty sure I know which repo something is in.)