Support a `--gmrc-path [filepath]` or `--config [filepath]` option

[ben-pr-p]

Feature description

I'd like to be able to run:

graphile-migrate --config apps/api/.gmrc

and

graphile-migrate --config apps/coordinator/.gmrc

in my monorepo which contains projects that communicate together but run on 2 separate databases and should have different DATABASE_URL's and different SHADOW_DATABASE_URL's

Breaking changes

Don't think we'd need any breaking changes - if's not present, we default back to .gmrc(.js).

Supporting development

I [tick all that apply]:

• [x] am interested in building this feature myself
• [x] am interested in collaborating on building this feature
• [ ] am willing to help testing this feature before it's released
• [ ] am willing to write a test-driven test suite for this feature (before it exists)
• [ ] am a Graphile sponsor ❤️
• [x] have an active support or consultancy contract with Graphile