Add support for shell completion (#340)

* Add build script to generate completion scripts

* Add installation doc for the shell auto-completion

polkadot/cli/Cargo.toml

@@ -3,9 +3,10 @@ name = "polkadot-cli"
 version = "0.2.0"
 authors = ["Parity Technologies <admin@parity.io>"]
 description = "Polkadot node implementation in Rust."
+build = "build.rs"
 
 [dependencies]
-clap = { version = "2.27", features = ["yaml"] }
+clap = { version = "~2.32", features = ["yaml"] }
 backtrace = "0.3"
 env_logger = "0.4"
 error-chain = "0.12"
@@ -42,3 +43,6 @@ polkadot-runtime = { path = "../runtime" }
 polkadot-service = { path = "../service" }
 polkadot-transaction-pool = { path = "../transaction-pool" }
 names = "0.11.0"
+
+[build-dependencies]
+clap = "~2.32"

polkadot/cli/build.rs

@@ -0,0 +1,43 @@
+#[macro_use]
+extern crate clap;
+
+use std::fs;
+use std::env;
+use clap::Shell;
+use std::path::Path;
+
+fn main() {
+	build_shell_completion();
+}
+
+/// Build shell completion scripts for all known shells
+/// Full list in https://github.com/kbknapp/clap-rs/blob/e9d0562a1dc5dfe731ed7c767e6cee0af08f0cf9/src/app/parser.rs#L123
+fn build_shell_completion() {
+	let shells = [Shell::Bash, Shell::Fish, Shell::Zsh, Shell::Elvish, Shell::PowerShell];
+	for shell in shells.iter() {
+    	build_completion(shell);
+	}
+}
+
+/// Build the shell auto-completion for a given Shell
+fn build_completion(shell: &Shell) {
+	let yml = load_yaml!("src/cli.yml");
+
+	let outdir = match env::var_os("OUT_DIR") {
+        None => return,
+        Some(dir) => dir,
+    };
+    let path = Path::new(&outdir)
+    	.parent().unwrap()
+    	.parent().unwrap()
+    	.parent().unwrap()
+    	.join("completion-scripts");
+
+    fs::create_dir(&path).ok();
+
+    let mut app = clap::App::from_yaml(&yml);
+    app.gen_completions(
+    	"polkadot",
+    	*shell,
+    	&path);
+}