Optional Named Placeholders?

I noticed your library is quite advanced with singular/plural- and boolean handling, can it also do something like this?

```
en-US: "SpeedKey", "You are driving {speedMph:0.0} Mph."
en-CA: "SpeedKey", "You are driving {speedKmh:0.0} km/h."
de-DE: "SpeedKey", "Sie fahren {speedKmh:0.0} km/h."

Label.Text = Tr("SpeedKey", { { "speedMph", speedMph }, { "speedKmh", speedKmh } });

```
