Remove useless features

[TheElectronWill]

• valueMap():

After a search of all the Github projects mentioning night-config, I've found that the only occurrences of config.valueMap() are for iterating on the config entries, keys or values. There are much simpler ways of providing iterations on entries, keys or values, therefore I could ditch valueMap. Since on-the-go conversion seems to be unused too, its dependencies like TransformingMap can be removed to make the library lighter.

• TransformingMap and ConversionTable:

At the very least they should be moved to another module.