Add Fish shell completion (streamnative#979)

Author: jeloba

docs/en/enable_completion.md

@@ -21,7 +21,7 @@
 
 ## Enabling shell autocompletion
 
-pulsarctl provides autocompletion support for Bash and Zsh, which can save you a lot of typing.
+pulsarctl provides autocompletion support for Bash, Zsh, and Fish, which can save you a lot of typing.
 
 ### Zsh
 
@@ -94,3 +94,17 @@ pulsarctl completion bash >/usr/local/etc/bash_completion.d/pulsarctl.bash
 echo 'source /usr/local/etc/bash_completion.d/pulsarctl.bash' >> ~/.bashrc
 source ~/.bashrc
 ```
+
+### Fish
+
+To load completions once in your current session run:
+		
+```bash
+pulsarctl completion fish | source
+```
+
+To load completions for each session, run:
+
+```bash
+pulsarctl completion fish > ~/.config/fish/completions/pulsarctl.fish
+```

pkg/ctl/completion/completion.go

@@ -65,6 +65,22 @@ rm -f ~/.zcompdump; compinit
 		},
 	}
 
+	var fishCompletionCmd = &cobra.Command{
+		Use:   "fish",
+		Short: "Generates Fish completion scripts",
+		Long: `To load completions run:
+
+pulsarctl completion fish | source
+
+		To load completions for each session, run:
+
+pulsarctl completion fish > ~/.config/fish/completions/pulsarctl.fish
+`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			return rootCmd.GenFishCompletion(os.Stdout, true)
+		},
+	}
+
 	cmd := &cobra.Command{
 		Use:   "completion",
 		Short: "Generates shell completion scripts",
@@ -77,6 +93,7 @@ rm -f ~/.zcompdump; compinit
 
 	cmd.AddCommand(bashCompletionCmd)
 	cmd.AddCommand(zshCompletionCmd)
+	cmd.AddCommand(fishCompletionCmd)
 
 	return cmd
 }