imp(Positional Arguments): now displays value name if appropriate

When value names are use, they will be displayed in help and error
messages instead of the argument name. For example, an argument named
`arg` but with the value name `FILE` will be displayed as `[FILE]`.
Likewise multiple value names will be displayed properly such as `[FILE1] [FILE2]`

Closes #420

src/args/arg_builder/positional.rs

@@ -130,11 +130,19 @@ impl<'n, 'e> PosBuilder<'n, 'e> {
 impl<'n, 'e> Display for PosBuilder<'n, 'e> {
     fn fmt(&self, f: &mut Formatter) -> Result {
         if self.settings.is_set(ArgSettings::Required) {
-            try!(write!(f, "<{}>", self.name));
+            if let Some(ref names) = self.val_names {
+                try!(write!(f, "{}", names.values().map(|n| format!("<{}>", n)).collect::<Vec<_>>().join(" ")));
+            } else {
+                try!(write!(f, "<{}>", self.name));
+            }
         } else {
-            try!(write!(f, "[{}]", self.name));
+            if let Some(ref names) = self.val_names {
+                try!(write!(f, "{}", names.values().map(|n| format!("[{}]", n)).collect::<Vec<_>>().join(" ")));
+            } else {
+                try!(write!(f, "[{}]", self.name));
+            }
         }
-        if self.settings.is_set(ArgSettings::Multiple) {
+        if self.settings.is_set(ArgSettings::Multiple) && self.val_names.is_none() {
             try!(write!(f, "..."));
         }