Allow MustLoadMessageFile to accept an optional langTag

[jtruong2]

MustLoadMessageFile() takes a path that follows the file naming convention of xxx.en.json and automatically parses the language tag. This incorrectly retrieves the language tag if the path does not follow the convention. In my case, the locales path is locales/en/translation.json. Passing this path to MustLoadMessageFile() will result in the langTag being translation instead of en. It'd be super helpful if either the function could also parse the aforementioned path or accept an optional langTag arg to override the automatic parsing.

[nicksnyder]

Yeah, I wish I had not as tightly coupled the locale detection to the file name. I'm not going to make backward incompatible changes to existing method signatures, so the only option would be to add a new method.

The best workaround is what I mentioned here: #376 (comment).