versioning cult-cargo: images versus schemas

[o-smirnov]

@SpheMakh @Athanaseus this thought was brought on by PR #7.

I propose we bump minor version numbers when images change. If only schemas change (as in #7), we only bump the patchlevel.

After #7 is merged, I propose pushing out release 0.1.0, and from then onwards sticking to this versioning scheme.