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/README.md

@@ -37,6 +37,7 @@ soulseek-rs message send|read
 soulseek-rs shares list|add|remove|status|reindex
 soulseek-rs config path|list|get|set
 soulseek-rs portmap                       # test automatic port mapping
+soulseek-rs completions install           # tab completion for bash/zsh/fish
 ```
 
 Add `--json` for newline-delimited JSON instead of tab-separated text, so