optparse-applicative 0.13.0

• Implement command groups, which allow subcommands to have their own
  usage description.
• Implement showHelpOnEmpty, which is similar to showHelpOnError, but only
  fires when a command or subcommand is begun, and suppresses the "Missing:"
  error text.
• Fix ghc 8.0 warnings
• Fix ghc 7.10 warnings
• Bump dependency bounds
• Add maybeReader function for convenient ReadM creation
• Move eitherReader to Readers section (for better discoverability)
• Fix hsubparser metavar override
• Remove ComplError, which was dead code.
• Reimplement Missing error generation, which overly complicated evalParser.
• Export Semigroup instances for types which are also Monoids. Removes
  mappend synonym (<>) export, as it clashes with Semigroup exports.
  One may need to import Data.Monoid or Data.Semigroup when upgrading.
• Use a Cabal test suite for tests, simplify test dependencies.