Add release engine for Composer

[ezekg]

Maybe use a php subdomain?

[ezekg]

One issue with Composer is that auth is per-domain, not per-package. So multiple Keygen users could not distribute packages to the same user, since the packages would require 2 different tokens to access.

One solution would be to allow an account subdomain scope. But Heroku doesn’t seem to play well with that many custom domains. It’d also require another wildcard cert if we wanted to scope it under <account>.php.. Heroku does support wildcard subdomains, so *.php.keygen.sh or *.get.keygen.sh would work.

A better solution may be custom domains?