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/commands/mod.rs

@@ -5,6 +5,7 @@
 //! first; the ones that only read the config file or the local machine are
 //! dispatched by `main::run` before any credentials are demanded.
 
+pub mod completions;
 pub mod peer;
 pub mod settings;
 pub mod skills;
@@ -196,7 +197,10 @@ pub fn run(ctx: &Ctx, command: Commands) -> CliResult {
         // These need no account, so main.rs answers them before asking for
         // credentials; the arms exist only to keep the match exhaustive.
         Commands::Portmap => portmap(&ctx.out, ctx.settings.listen_port),
-        Commands::Config(_) | Commands::Shares(_) | Commands::Skills(_) => {
+        Commands::Config(_)
+        | Commands::Shares(_)
+        | Commands::Skills(_)
+        | Commands::Completions(_) => {
             Err(CliError::usage("handled without a session; see main::run"))
         }
     }