feat: install tab completion for bash, zsh, and fish (#16)

`completions install` generates the script from the same clap definition
the binary parses with, so it cannot drift from `--help`, and writes it
where each shell already looks. fish scans its own completions directory
unprompted; bash and zsh get one marked `source` line appended to their rc
file, because nothing puts a per-user directory on their search path by
default. `completions uninstall` takes back exactly those lines and leaves
the rest of the file alone.

soulseek-rs/src/output.rs

@@ -445,6 +445,28 @@ impl Record for SkillRecord {
     }
 }
 
+/// What happened to one shell's completion script.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
+pub struct CompletionRecord {
+    /// `bash`, `zsh`, or `fish`.
+    pub shell: String,
+    pub path: String,
+    /// `installed` (written), `unchanged` (already identical), `removed`, or
+    /// `absent`.
+    pub action: String,
+}
+
+impl Record for CompletionRecord {
+    fn text(&self) -> String {
+        format!(
+            "{}\t{}\t{}",
+            sanitize(&self.action),
+            sanitize(&self.shell),
+            sanitize(&self.path)
+        )
+    }
+}
+
 /// The result of probing automatic port mapping.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize)]
 pub struct PortmapRecord {