Use Number instead of Int for choice arguments

[theisenp]

Similar to #215. The underlying ICU library supports decimal numbers for choice arguments, so we should too.

{0, choice, 0#Zero | 0.5#Half | 1<{0,number}}

The official documentation discourages the use of choice arguments in favor of plural, select, etc so in practice people shouldn't hit this too often. But it's an easy fix so why not.

Partially addresses #207.