Add tool config

[mitsuhiko]

With requests like #278 and my desire to make cargo-insta test support nextest it might be time to introduce a config file. A potential location could be workspace-root/.config/cargo-insta.yaml or potentially even insta.yaml if that config file should be used for insta.

Some of the options to consider would be:

• Use --accept-unseen to be a default
• Use --review by default
• Enable snapshot compression
• Switch between cargo test and cargo nextest as default

[repi]

support for cargo nextest through config (or env var?) would be lovely, we generally use it for our tests in our large repo as is massively faster but do want to be able to run it with cargo insta --review for fast update and review of snapshots.

[mitsuhiko]

Yap. I absolutely want to get this in. I'm using it myself now quite a bit too.

[mitsuhiko]

The tool config is in, the remaining tasks are tracked as part of #323