make hooks messages configurable

...and update them to reflect current usage

why:
- these messages appeared to be just wrong (referencing `aide` instead of firstaide or an `update` command that doesn't appear to exist)
- and in any case it's nice to have them all customizeable not just the getting_started message. based on the presence of the `Messages` struct I presume this was always intended

how:
- add fields to `Messages` struct, named based on the hook they are associated with
- set sensible defaults for messages
- add messages to the hooks scripts
- splat configured messages into hooks scripts based on how it was done for the getting_started message

docs: updated the readme to document messages configuration

validation: tested new message configurations in a different project, `cargo test`

src/config.rs

@@ -51,8 +51,11 @@ impl AsRef<Path> for ParentDir {
 
 #[derive(Debug, Deserialize)]
 pub struct Messages {
+    #[serde(default)]
     pub getting_started: String,
+    #[serde(default)]
     pub stale: String,
+    #[serde(default)]
     pub inactive: String,
 }