Slightly better ergonomics for custom format override

[yiminghan]

WHY?

Why I understand you can do "timeago.setLocaleMessages('en', CustomEnglishMessages());` to override custom messages for maybe that one specific usecase, the developer has to remember to reset the locale after and it's error prune.

This PR proposes a slightly better way to use custom message formats - just pass them into .format()

This should unlock slightly easier ways for those specific use cases that may not apply to the entire application