Support multiple repositories per execution

- Add RepoConfig type for per-repo path/remote/branch
- Options now holds [RepoConfig] instead of single repo
- YAML config file support (--config) for multi-repo setup
- --repo still works for quick single-repo usage
- Concurrent watchers via async (one thread per repo)
- --debounce flag overrides config file value

converge.example.yaml

@@ -0,0 +1,12 @@
+# Example converge configuration file.
+# Usage: converge --config converge.yaml
+
+repos:
+  - path: /home/jbrechtel/org
+    remote: origin
+    branch: main
+  - path: /work/personal/hawat
+    # remote defaults to "origin"
+    # branch defaults to "main"
+
+# debounce_ms: 5000   # optional, defaults to 5000