Make the validation especific to str plugin for now.

src/plugin.rs

@@ -5,14 +5,6 @@ use std::io;
 pub trait Plugin {
     fn config(&mut self) -> Result<CommandConfig, ShellError>;
 
-    #[allow(unused)]
-    fn is_valid(&self) -> bool { 
-        true
-    }
-
-    #[allow(unused)]
-    fn log_error(&mut self, message: &str) { }
-
     #[allow(unused)]
     fn begin_filter(&mut self, call_info: CallInfo) -> Result<Vec<ReturnValue>, ShellError> {
         Ok(vec![])

src/plugins/str.rs

@@ -21,6 +21,14 @@ impl Str {
         }
     }
 
+    fn is_valid(&self) -> bool {
+        (self.downcase && !self.upcase) || (!self.downcase && self.upcase)
+    }
+
+    fn log_error(&mut self, message: &str) {
+        self.error = Some(message.to_string());
+    }
+
     fn to_downcase(&mut self) {
         self.downcase = true;
 
@@ -107,14 +115,6 @@ impl Plugin for Str {
         })
     }
 
-    fn is_valid(&self) -> bool {
-        (self.downcase && !self.upcase) || (!self.downcase && self.upcase)
-    }
-
-    fn log_error(&mut self, message: &str) {
-        self.error = Some(message.to_string());
-    }
-
     fn begin_filter(&mut self, call_info: CallInfo) -> Result<Vec<ReturnValue>, ShellError> {
         if call_info.args.has("downcase") {
             self.to_downcase();