Extended Try RFC

[Rendered view.](https://github.com/TyOverby/rfcs/blob/extended_try/active/0000-extended-try.md)
@nick29581 The existing `try!()` can easily be extended to take two arguments without breaking any backwards compatibility. The suggestion for a new macro is only as a potential readability aid. Personally, I think using a new macro is unnecessary, providing a second arg to `try!()` should be visible enough.