examples: Add AppSettings example

examples/16_AppSettings.rs

@@ -0,0 +1,31 @@
+extern crate clap;
+
+use clap::{App, AppSettings, SubCommand};
+
+fn main() {
+    // You can use AppSettings to change the application level behavior of clap. .setting() function
+    // of App struct takes AppSettings enum as argument. There is also .settings() function which
+    // takes slice of AppSettings enum.  You can learn more about AppSettings in the documentation,
+    // which also has examples on each setting.
+    //
+    // This example will only show usage of one AppSettings setting. See documentation for more
+    // information.
+
+    let matches = App::new("myapp")
+                        .setting(AppSettings::SubcommandsNegateReqs)
+                                            // Negates requirement of parent command.
+
+                        .arg_from_usage("<input> 'input file to use'")
+                                            // Required positional argument called input.  This
+                                            // will be only required if subcommand is not present.
+
+                        .subcommand(SubCommand::with_name("help")
+                                                .about("shows help message"))
+                                            // if program is invoked with subcommand, you do not
+                                            // need to specify the <input> argument anymore due to
+                                            // the AppSettings::SubcommandsNegateReqs setting.
+
+                        .get_matches();
+
+    // Contiued program logic goes here...
+}