feat(glossy): Check validity of modifiers

When an unrecognized modifier is applied, glossy should panic to warn
the user, instead of silently ignoring the modifier. This might catch
potential misspellings of modifiers.

glossy/README.md

@@ -411,3 +411,5 @@ This project is licensed under the MIT License.
   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).
+- **Breaking**: When an unrecognized modifier is supplied with the term,
+  glossy will now panic instead of ignoring the modifier.