imp(srs/args): Added longest_filter to AnyArg trait

This function allows providing an extra filter to remove elements when finding
the longest element.

src/app/mod.rs

@@ -936,6 +936,7 @@ impl<'n, 'e> AnyArg<'n, 'e> for App<'n, 'e> {
     fn takes_value(&self) -> bool { true }
     fn help(&self) -> Option<&'e str> { self.p.meta.about }
     fn default_val(&self) -> Option<&'n str> { None }
+    fn longest_filter(&self) -> bool { true }
 }
 
 impl<'n, 'e> fmt::Display for App<'n, 'e> {

src/args/any_arg.rs

@@ -25,6 +25,7 @@ pub trait AnyArg<'n, 'e> {
     fn val_names(&self) -> Option<&VecMap<&'e str>>;
     fn help(&self) -> Option<&'e str>;
     fn default_val(&self) -> Option<&'n str>;
+    fn longest_filter(&self) -> bool;
 }
 
 pub trait DispOrder {

src/args/arg_builder/flag.rs

@@ -129,6 +129,7 @@ impl<'n, 'e> AnyArg<'n, 'e> for FlagBuilder<'n, 'e> {
     fn val_delim(&self) -> Option<char> { None }
     fn help(&self) -> Option<&'e str> { self.help }
     fn default_val(&self) -> Option<&'n str> { None }
+    fn longest_filter(&self) -> bool { self.long.is_some() }
 }
 
 impl<'n, 'e> DispOrder for FlagBuilder<'n, 'e> {

src/args/arg_builder/option.rs

@@ -193,6 +193,7 @@ impl<'n, 'e> AnyArg<'n, 'e> for OptBuilder<'n, 'e> {
     fn takes_value(&self) -> bool { true }
     fn help(&self) -> Option<&'e str> { self.help }
     fn default_val(&self) -> Option<&'n str> { self.default_val }
+    fn longest_filter(&self) -> bool { true }
 }
 
 impl<'n, 'e> DispOrder for OptBuilder<'n, 'e> {

src/args/arg_builder/positional.rs

@@ -180,6 +180,7 @@ impl<'n, 'e> AnyArg<'n, 'e> for PosBuilder<'n, 'e> {
     fn takes_value(&self) -> bool { true }
     fn help(&self) -> Option<&'e str> { self.help }
     fn default_val(&self) -> Option<&'n str> { self.default_val }
+    fn longest_filter(&self) -> bool { true }
 }
 
 impl<'n, 'e> DispOrder for PosBuilder<'n, 'e> {