Fix unreachable patterns in command macro

When attempting to use a named argument with a type of String, the
framework will try to parse the received argument - a String - into a
String. This can never fail, so Rust warns that the `Err` arm of the
`match` can never be reached.

Let's just ignore that and everything will be fine.

src/ext/framework/mod.rs

@@ -140,7 +140,7 @@ macro_rules! command {
         }
     };
     ($fname:ident($c:ident, $m:ident, $a:ident, $($name:ident: $t:ty),*) $b:block) => {
-        #[allow(unused_mut)]
+        #[allow(unreachable_patterns, unused_mut)]
         pub fn $fname(mut $c: &mut $crate::client::Context, $m: &$crate::model::Message, $a: Vec<String>) -> ::std::result::Result<(), String> {
             let mut i = $a.iter();
             let mut arg_counter = 0;