Add support for fallback values in input prompts

[msanders]

This allows writing the following:

let proceed = Input.readBool(prompt: "Proceed? [Y/n]: ", defaultValue: true)

It falls back to defaultValue only for empty input. Invalid input is treated the same as it is already (i.e., it's refused until corrected, which matches the behavior of most CLIs).

Existing behavior is the same (no breaking changes).

[jakeheis]

This looks great, thanks for the contribution! Would ideally like to have a test in InputTests.swift before merging this

[msanders]

Thanks, done.