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
- use serde default attributes to allow mixing defaults and custom messages without errors
- 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

@@ -51,21 +51,33 @@ impl AsRef<Path> for ParentDir {
 
 #[derive(Debug, Deserialize)]
 pub struct Messages {
+    #[serde(default = "default_getting_started")]
     pub getting_started: String,
+    #[serde(default = "default_stale")]
     pub stale: String,
+    #[serde(default = "default_inactive")]
     pub inactive: String,
 }
-
 impl Default for Messages {
     fn default() -> Self {
         Self {
-            getting_started: "firstaide --help".into(),
-            stale: "firstaide build".into(),
-            inactive: "build firstaide and run \"firstaide build\"".into(),
+            getting_started: default_getting_started(),
+            stale: default_stale(),
+            inactive: default_inactive(),
         }
     }
 }
 
+fn default_getting_started() -> String {
+    "firstaide --help".to_string()
+}
+fn default_stale() -> String {
+    "firstaide build".to_string()
+}
+fn default_inactive() -> String {
+    "build firstaide and run \"firstaide build\"".to_string()
+}
+
 impl Config {
     pub fn load<T: Into<PathBuf>>(dir: Option<T>) -> Result<Config> {
         let dir = match dir {