Fix !!convert command to work for 'c' and 'f'

[Trasiva]

At present when you try to do something like !!convert 38c to f or !!convert 80f to c, Cap will error because she doesn't recognize them as valid temperature variants. She'll work for C, Celsius, F, and Fahrenheit, so if we could include lowercase, that'd be great. Thanks Zirak bby!