Basic parsing support for spelled-out numbers

Add support for parsing spelled-out numbers in expressions like "in five hours". By default, only the numbers 0 till 20 are recognized, but support is extended up to 100 when icu is available.

Unfortunately I didn't read your implementation notes until I finished this implementation, so it's a bit different (less advanced) from what you proposed there. I understand if you don't want to merge half an implementation, but I think it's a nice start.