Add prefix customization command

src/framework/commands/arguments.rs

@@ -31,6 +31,10 @@ impl FromArguments for String {
     }
 }
 
+/// Consumes the entire remaining string as a single argument. If no string is remaining, it
+/// returns an empty string.
+///
+/// Parsing this argument type never fails.
 pub struct GreedyString(pub String);
 
 impl FromArguments for GreedyString {