Add support for typescript (ts and tsx)

[fermuch]

After reading #128, I looked around the code of msgfmt:extract and found it's using regexes for parsing files and file extensions.
Since typescript augments vanilla JS, there should be no change to how the regexes for JS/JSX are handled, but I'm a newbie to typescript and might be missing something.

I tried locally to just add ts and tsx extensions, reuse the same parsers of JS and JSX, and it works perfectly in my project. See https://github.com/fermuch/meteor-messageformat for the changes.

Is it okay to add support this way for TS/TSX? If it is, I can rework those changes to create a PR.
Maybe someone else with more knowledge about TS/TSX can point me if there's something I'm missing? CC @tomitrescak