add completion subcommands (#99)

* add completion subcommands

* improve error mgmt, add entry to readme

Co-authored-by: Clément DOUIN <soywod@users.noreply.github.com>

src/completion/cli.rs

@@ -0,0 +1,34 @@
+use clap::{self, App, ArgMatches, SubCommand};
+use error_chain::error_chain;
+use std::io;
+
+error_chain! {}
+
+pub fn completion_subcmds<'a>() -> Vec<App<'a, 'a>> {
+    vec![
+        SubCommand::with_name("bash-completions").about("Generates bash completions script"),
+        SubCommand::with_name("zsh-completions").about("Generates zsh completions script"),
+        SubCommand::with_name("fish-completions").about("Generates fish completions script"),
+    ]
+}
+
+pub fn completion_matches(mut app: App, matches: &ArgMatches) -> Result<bool> {
+    use clap::Shell::*;
+
+    if matches.is_present("bash-completions") {
+        app.gen_completions_to("himalaya", Bash, &mut io::stdout());
+        return Ok(true);
+    }
+
+    if matches.is_present("zsh-completions") {
+        app.gen_completions_to("himalaya", Zsh, &mut io::stdout());
+        return Ok(true);
+    }
+
+    if matches.is_present("fish-completions") {
+        app.gen_completions_to("himalaya", Fish, &mut io::stdout());
+        return Ok(true);
+    }
+
+    Ok(false)
+}