Implement configurable policies

[bradleyfalzon]

Currently abicheck is focused on https://golang.org/doc/go1compat - but others might have other philosophies, or may want a strict mode but for it to raise warnings.

Ideally, you'd be able to customise each type of error, whether it's OK, Non Breaking or Breaking, but it maybe simpler for v1 to have either strict or go1compat mode.

At the very least, this needs to be documented.