Add support for validation of fluent resources to cargo-i18n command line tool

[kellpossible]

Ideas for validations:

• Check that all languages are able to be loaded
• Check for duplicate keys
• Check that args match across different languages for the same message.
• Report for the percent translated.
• Check that the message format parses correctly
• Check for messages in translated languages which are not present in the fallback language, and are probably redundant.
• For projects exclusively using the fl!() macro, a check to see when messages are no longer being used (and an option to ignore the warning based on a special comment keyword on the message) for exceptions where the macro isn't used.