feat(Groups): one can now specify groups which require AT LEAST one o…

…f the args

Unlike the previous ArgGroups, which made all args in the group mutually exclusive, one can now use
`ArgGroup::multiple(true)` which allows using more than one arg in the group (i.e. they're not all
mutually exclusive). When combined with `ArgGroup::required(true)` this effectively says, "At least
one of the args must be used, and using morethan one is also OK."

Closes #533

src/app/macros.rs

@@ -91,8 +91,10 @@ macro_rules! _handle_group_reqs{
             if found {
                 vec_remove_all!($me.required, &grp.args);
                 debugln!("Adding args from group to blacklist...{:?}", grp.args);
-                $me.blacklist.extend(&grp.args);
-                vec_remove!($me.blacklist, &$arg.name());
+                if !grp.multiple {
+                    $me.blacklist.extend(&grp.args);
+                    vec_remove!($me.blacklist, &$arg.name());
+                }
             }
         }
     })

src/args/group.rs

@@ -61,6 +61,8 @@ pub struct ArgGroup<'a> {
     pub requires: Option<Vec<&'a str>>,
     #[doc(hidden)]
     pub conflicts: Option<Vec<&'a str>>,
+    #[doc(hidden)]
+    pub multiple: bool,
 }
 
 impl<'a> ArgGroup<'a> {
@@ -81,6 +83,7 @@ impl<'a> ArgGroup<'a> {
             args: vec![],
             requires: None,
             conflicts: None,
+            multiple: false,
         }
     }
 
@@ -140,6 +143,26 @@ impl<'a> ArgGroup<'a> {
         self
     }
 
+    /// Allows more than one of the ['Arg']s in this group to be used. (Default: `false`)
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use clap::{ArgGroup, Arg};
+    /// let cfg_arg = Arg::with_name("config");
+    /// let in_arg = Arg::with_name("input");
+    /// // ...
+    /// ArgGroup::with_name("files")
+    ///     .args(&["config", "input"])
+    ///     .multiple(true)
+    /// # ;
+    /// ```
+    /// ['Arg']: ./struct.Arg.html
+    pub fn multiple(mut self, m: bool) -> Self {
+        self.multiple = m;
+        self
+    }
+
     /// Sets the group as required or not. A required group will be displayed in the usage string
     /// of the application in the format `[arg|arg2|arg3]`. A required `ArgGroup` simply states
     /// that one, and only one argument from this group *must* be present at runtime (unless
@@ -310,6 +333,7 @@ impl<'a, 'z> From<&'z ArgGroup<'a>> for ArgGroup<'a> {
             args: g.args.clone(),
             requires: g.requires.clone(),
             conflicts: g.conflicts.clone(),
+            multiple: g.multiple,
         }
     }
 }
@@ -507,6 +531,7 @@ impl<'a> Clone for ArgGroup<'a> {
             args: self.args.clone(),
             requires: self.requires.clone(),
             conflicts: self.conflicts.clone(),
+            multiple: self.multiple,
         }
     }
 }