Allow for overriding publishing conditions

The conditions used now for publishing sizes are tightly coupled to `env-ci` and the `master` branch.

See: https://github.com/GoogleChromeLabs/size-plugin/blob/master/src/publish-size.js#L21

It would be nice to support a custom logic there for repositories which use CI that are not recognized by `env-ci` (e.g. Jenkins) or use a different branch name than `master` (e.g. `main` / `develop`).