WIP add binary input support

This adds support for taking a url as input for a binary chunk,
instead of cloning from a git repo.

In definitions, these new fields are supported:

url: specifies the place to look for the binary
sha256: if present, specifies the expected sha256 for the binary
md5: if present, specifies the expected md5 for the binary

In config, these new fields are supported:
auth-user: (optional) user name for https authentication of url
auth-password: (optional) password for https authentication of url

The auth-user and auth-password should normally be exported as
environment variables YBD_auth_user and YBD_auth_password