Clap: Set ArgRequiredElseHelp option

This ensures that the help is printed (with a non-0 exit code) if no
command is specified.

src/main.rs

@@ -20,7 +20,7 @@ use std::{env, path::PathBuf, process};
 use ansi_term::{Color, Style};
 use app_dirs::AppInfo;
 use atty::Stream;
-use clap::Clap;
+use clap::{AppSettings, Clap};
 #[cfg(not(target_os = "windows"))]
 use pager::Pager;
 
@@ -53,6 +53,8 @@ const PAGER_COMMAND: &str = "less -R";
 
 #[derive(Clap, Debug)]
 #[clap(about = "A fast TLDR client", author, version)]
+#[clap(setting = AppSettings::ArgRequiredElseHelp)]
+#[clap(setting = AppSettings::HelpRequired)]
 struct Args {
     /// The command to show (e.g. `tar` or `git log`)
     #[clap(min_values = 1)]