Use Number instead of Int for plural arguments

[theisenp]

Currently we require Int for plural arguments, but the underlying ICU library supports formatting with Long, Double, etc.

There are plenty of use cases where this is valid, so it should be supported by Paraphrase. For example:

{length, plural,
  one {The race is 1 mile long}
  other {The race is # miles long}
}

// The race is 26.2 miles long

Partially addresses #207