Output formatting improvements

[djmattyg007]

Currently the output doesn't produce nice human-readable descriptions, and doesn't differentiate between singular and plural values.

❯ target/release/cpc '123 kph'
123 KilometersPerHour

❯ target/release/cpc '1 kph'
1 KilometersPerHour

It would be good if the output of the above was 123 kilometres per hour and 1 kilometre per hour.

[probablykasper]

Would be nice for sure. I think the way to go is implement fmt::Display for Unit