Set 'documentation' and 'hyperlink-source' to True by default.

Note that this only affects 'cabal-install', not Cabal itself.
Add a non-throwing variant of 'requireProgramVersion'.
Formatting, 80-col violations.
'D.S.Haddock': warn instead of dying when 'hscolour' isn't installed.

This means that if 'hyperlink-source' is set to True, but the 'HsColour'
executable is not present, documentation will be still generated.