cmd/loop: enable auto-completion

Save the output of "loop completion bash" to ~/.bash_completion.d/loop and
restart bash or do ". ~/.bash_completion.d/loop".

Author: starius

README.md

@@ -64,6 +64,32 @@ To execute a Loop In:
 loop in <amt_in_satoshis>
 ```
 
+### Bash completion
+
+To enable command line auto-completion for `loop` command, do this:
+
+```
+$ mkdir -p ~/.local/share/bash-completion/completions/
+$ loop completion bash > ~/.local/share/bash-completion/completions/loop
+$ . ~/.local/share/bash-completion/completions/loop
+$ echo '. ~/.local/share/bash-completion/completions/loop' >> ~/.bashrc
+```
+
+It will complete commands and flags if you type `loop ` and press TAB:
+```
+$ loop static <TAB>
+help             listswaps        new
+in               listunspent      summary
+listdeposits     listwithdrawals  withdraw
+
+$ loop static in --<TAB>
+--all              --label            --payment_timeout  --utxo
+--amount           --last_hop         --route_hints
+```
+
+It also supports zsh, fish, and powershell. Just replace "bash" with your shell
+name in the command above and ajust the completion file location.
+
 ### More info
 For more information about using Loop checkout our [Loop FAQs](./docs/faqs.md).
 

cmd/loop/main.go

@@ -94,7 +94,6 @@ var (
 )
 
 const (
-
 	// satAmtFmt formats a satoshi value into a one line string, intended to
 	// prettify the terminal output. For Instance,
 	// 	fmt.Printf(f, "Estimated on-chain fee:", fee)
@@ -163,9 +162,10 @@ func fatal(err error) {
 
 func main() {
 	rootCmd := &cli.Command{
-		Name:    "loop",
-		Usage:   "control plane for your loopd",
-		Version: loop.RichVersion(),
+		Name:                  "loop",
+		Usage:                 "control plane for your loopd",
+		Version:               loop.RichVersion(),
+		EnableShellCompletion: true,
 		Flags: []cli.Flag{
 			&cli.StringFlag{
 				Name:    "rpcserver",