Implement cli option for custom derive

closes rust-lang#2170

Author: hcldan

bindgen-cli/options.rs

@@ -225,6 +225,12 @@ where
                     "Create PartialEq implementation, if it can not be derived \
                      automatically.",
                 ),
+            Arg::new("with-derive-custom")
+                .long("with-derive-custom")
+                .help("Derive custom traits on any type.")
+                .value_name("trait")
+                .multiple_occurrences(true)
+                .number_of_values(1),
             Arg::new("with-derive-default")
                 .long("with-derive-default")
                 .help("Derive Default on any type."),
@@ -744,6 +750,21 @@ where
         builder = builder.impl_partialeq(true);
     }
 
+    if let Some(traits) = matches.values_of("with-derive-custom") {
+        #[derive(Debug)]
+        struct CustomTraitCallback {
+            derives: Vec<String>
+        }
+        impl bindgen::callbacks::ParseCallbacks for CustomTraitCallback {
+            fn add_derives(&self, _name: &str) -> Vec<String> {
+                self.derives.to_owned()
+            }
+        }
+        
+        let derives: Vec<String> = traits.into_iter().map(|t| t.to_string()).collect();
+        builder = builder.parse_callbacks(Box::new(CustomTraitCallback { derives }));
+    }
+
     if matches.is_present("with-derive-default") {
         builder = builder.derive_default(true);
     }