Retrieve
getArgs, and attempt to parse it into a valid value of
an
Options type plus a list of left-over arguments. The options
and arguments are then passed to the provided computation.
If parsing fails, this computation will print an error and call
exitFailure.
If parsing succeeds, and the user has passed a
--help flag,
and the developer is using the default help flag definitions, then
this computation will print documentation and call
exitSuccess.
See
runSubcommand for details on subcommand support.