Improved the parsing of `-XExtension` flags.

The `Read LanguageExt` instance is now used to convert strings into
`LanguageExt`s, and passing -XNonexistentExtension results in an error
instead of attempting to load the file "-XNonexistentExtension".
Merge pull request #768 from trillioneyes/extension-checking

Improved the parsing of `-XExtension` flags.