Make 'trueArg'/'falseArg' create a 'BoolOpt' instead of 'ChoiceOpt'.

'ChoiceOpt' is really an enum, when we were using it is a boolean the default
value wasn't printed correctly in the config file.

Before:

-- hscolour:

After:

-- hscolour: False
Support 'haddock' in 'cabal.config'.
Formatting.