feat(glossy): Panic on multiple requested modes

When multiple modes are requested at the same time
(like 'long' & 'short'), panic with a clear error message.

This is better than guessing what the user wanted.

glossy/README.md

@@ -409,3 +409,5 @@ This project is licensed under the MIT License.
   in your custom theme to keep the previous behaviour.
 - **Breaking**: The `both` mode (previously called modifier) by default
   now counts towards the first use of a term.
+- **Breaking**: When multiple conflicting modes are supplied (like 'short'
+  and 'long'), glossy now throws an error (with a clear message).