Add support for allowing orgs from Github/Gitlab/Bitbucket

[Jake-Shadle]

Generally, I think most projects in the Rust space probably follow a similar procedure as we do when they want to fix a bug or add a feature to one of their dependencies, which is basically.

1. Fork the crate to your/your company's "organization" (organization in Github terminology, might be different for other providers)
2. Hack away locally, probably just patching your project(s) to use a path dependency to the cloned fork
3. Push changes to your fork, and once you're happy, change the path dependency to a git dependency and point it to your fork for others/CI to be able to use the same changes easily
4. Eventually (hopefully!) make a PR to the original repo with your changes
5. Hopefully get your changes merged to the original repo
6. Wait until a release is made that incorporates your changes, possibly changing the git source to point to the original repo
7. Remove the git source and instead point at the new version of the crate with your changes
8. Profit!

This process is generally great, but because of the sources check, it gets kind of tedious, for example, here is what our largest project looks like right now.

allow-git = [
    "https://github.com/EmbarkStudios/cpal",
    "https://github.com/EmbarkStudios/winit",
    "https://github.com/EmbarkStudios/sentry-rust",
    "https://github.com/EmbarkStudios/lmdb-rs",
    "https://github.com/EmbarkStudios/juniper",
    "https://github.com/EmbarkStudios/rust-postgres",
    "https://github.com/EmbarkStudios/vk-mem-rs",
]

That's way too many duplicated characters!

Instead, we should just have something like

[sources.allow-org]
github = [
    "EmbarkStudios",
]

It seems like Gitlab and Bitbucket would also work with this, so those could be added as well, any other providers that followed the same <domain>/<org>/... scheme could be added in the future.