feat: flag and config to skip notifications by jlucktay (#5)

* feat: flag and config to skip notifications

At the end of a run, topgrade normally sends a system notification.
This change adds a command-line flag and a config-file option to
disable this behaviour.

* fix: clippy issues

Also derive Eq where PartialEq is being derived.
https://rust-lang.github.io/rust-clippy/master/index.html#derive_partial_eq_without_eq

Authored-by: James Lucktaylor <jlucktay+github@gmail.com>
Approved-by: Thomas Schönauer <t.schoenauer@hgs-wt.at>

config.example.toml

@@ -37,6 +37,9 @@
 # Cleanup temporary or old files
 #cleanup = true
 
+# Skip sending a notification at the end of a run
+#skip_notify = true
+
 [git]
 #max_concurrency = 5
 # Additional git repositories to pull