split ci files

[aboedo]

CircleCI doesn't have direct support for splitting a config file into multiple, so this adds support for it a bit more manually:

It adds logic to our pre-commit hook to concatenate files in a configuration folder. This only runs if files in that folder have changed.

This is the basic file structure:

So all of the files in that folder, in order, comprise the config.yml file that looks pretty much identical to the one we had before.

This approach does take two assumptions:

1. developers will understand that they shouldn't edit the config.yml file directly (there's a warning in the file itself, and we could get fancier and detect this with git I suppose?)
2. developers will use the pre-commit hook (which... we should in any case, there's a reason we have it)

[aboedo]

@joshdholtz tagged you for early thoughts since you had had a stab at this in the past

[aboedo]

the diff makes this look big but there really are only 2 small files worth reviewing in here, the rest are mostly ignorable in they're cut / paste from the original config.yml file