Add remains, an optional alternative to rest. And fix a typo in t…

…he `len` docs (#636)

src/framework/standard/args.rs

@@ -737,13 +737,21 @@ impl Args {
     /// Starting from the offset, return the remainder of available arguments.
     #[inline]
     pub fn rest(&self) -> &str {
+        self.remains().unwrap_or_default()
+    }
+
+    /// Starting from the offset, return the remainder of available arguments.
+    ///
+    /// Returns `None` if there are no remaining arguments.
+    #[inline]
+    pub fn remains(&self) -> Option<&str> {
         if self.is_empty() {
-            return "";
+            return None;
         }
 
         let (start, _) = self.span();
 
-        &self.message[start..]
+        Some(&self.message[start..])
     }
 
     /// Return the full amount of recognised arguments.
@@ -752,7 +760,7 @@ impl Args {
     /// # Note
     ///
     /// The value returned is to be assumed to stay static.
-    /// However, if `find` was called previously, and was succesful, then the value is substracted by one.
+    /// However, if `find` was called previously, and was successful, then the value is substracted by one.
     #[inline]
     pub fn len(&self) -> usize {
         self.args.len()