Add a non-throwing variant of 'requireProgramVersion'.
Formatting, 80-col violations.
'cabal 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.

Note that 'cabal hscolour' will still exit with error in this case.