Add support for a configuration file that can contain the patterns, etc. and be included in the repo is desired.

It would be very useful to support a config file, that can be pointed at (`--config=./git-secrets.yaml`), which can contain all configuration settings, including patterns. This way it can be included in the repo (if desired) and run via tools like `pre-commit` in a way that guarantees that the tool is configured properly, without requiring extra git config. This is especially useful in a CI/CD workflow.

Start by tracing how git-secrets currently reads configuration and patterns, then compare that path with the --config=./git-secrets.yaml entry point described here. Define how repository-shipped settings should be loaded and applied for pre-commit and CI/CD, and verify that patterns and other configuration can be supplied without extra git config.

Written by the indexing model from the issue text.