Allow update of config value during validation

[larskuhtz]

Sometimes validation of a configuration value fails, but the failure could be resolved, for example, from other configuration settings. To allow this the type of ConfigValidation should be changed from

type ConfigValidation α λ = (MonadIO μ, Functor μ, Applicative μ, MonadError Text μ, MonadWriter (λ Text) μ) => α -> μ ()

to

type ConfigValidation α λ = (MonadIO μ, Functor μ, Applicative μ, MonadError Text μ, MonadWriter (λ Text) μ) => α -> μ α

and the resulting value should be passed as configuration to the application.

[larskuhtz]

This feature would be an API change and should go into version 0.3. I don't think that there it is worth to do it in a backward compatible way. Also, the change in the function type should prevent compatibility bugs that wouldn't show up during compilation and would result in bad run-time behavior.