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/main.rs

@@ -64,6 +64,9 @@ fn run(mut cli: Cli, out: &Out) -> CliResult {
         Commands::Skills(ref command) => {
             return commands::skills::run(out, command);
         }
+        Commands::Completions(ref command) => {
+            return commands::completions::run(out, command);
+        }
         Commands::Config(ConfigCommand::Path) => {
             return commands::settings::config_path(out, &store);
         }