feat(Settings): Add unset_setting and unset_settings fns to App (#598)

Closes #590.

src/app/mod.rs

@@ -484,6 +484,47 @@ impl<'a, 'b> App<'a, 'b> {
         self
     }
 
+    /// Disables a single command, or [`SubCommand`], level setting.
+    /// 
+    /// See [`AppSettings`] for a full list of possibilities and examples.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// # use clap::{App, AppSettings};
+    /// App::new("myprog")
+    ///     .unset_setting(AppSettings::ColorAuto)
+    /// # ;
+    /// ```
+    /// [`SubCommand`]: ./struct.SubCommand.html
+    /// [`AppSettings`]: ./enum.AppSettings.html
+    pub fn unset_setting(mut self, setting: AppSettings) -> Self {
+        self.p.unset(setting);
+        self
+    }
+
+    /// Disables multiple command, or [`SubCommand`], level settings.
+    ///
+    /// See [`AppSettings`] for a full list of possibilities and examples.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// # use clap::{App, AppSettings};
+    /// App::new("myprog")
+    ///     .unset_settings(&[AppSettings::ColorAuto,
+    ///                       AppSettings::AllowInvalidUtf8])
+    /// # ;
+    /// ```
+    /// [`SubCommand`]: ./struct.SubCommand.html
+    /// [`AppSettings`]: ./enum.AppSettings.html
+    pub fn unset_settings(mut self, settings: &[AppSettings]) -> Self {
+        for s in settings {
+            self.p.unset(*s);
+        }
+        self 
+    }
+
     /// Sets the terminal width at which to wrap help messages. Defaults to `120`.
     ///
     /// `clap` automatically tries to determine the terminal width on Unix, Linux, and OSX if the

tests/app_settings.rs

@@ -247,3 +247,24 @@ fn leading_double_hyphen_trailingvararg() {
     assert!(m.is_present("opt"));
     assert_eq!(m.values_of("opt").unwrap().collect::<Vec<_>>(), &["--foo", "-Wl", "bar"]);
 }
+
+#[test]
+fn test_unset_setting() {
+    let m = App::new("unset_setting");
+    assert!(m.p.is_set(AppSettings::AllowInvalidUtf8));
+
+    let m = m.unset_setting(AppSettings::AllowInvalidUtf8);
+    assert!(!m.p.is_set(AppSettings::AllowInvalidUtf8));
+}
+
+#[test]
+fn test_unset_settings() {
+    let m = App::new("unset_settings");
+    assert!(&m.p.is_set(AppSettings::AllowInvalidUtf8));
+    assert!(&m.p.is_set(AppSettings::ColorAuto));
+    
+    let m = m.unset_settings(&[AppSettings::AllowInvalidUtf8,
+                               AppSettings::ColorAuto]);
+    assert!(!m.p.is_set(AppSettings::AllowInvalidUtf8));
+    assert!(!m.p.is_set(AppSettings::ColorAuto));
+}