Define option with flags, description and optional coercion fn.

The flags string should contain both the short and long flags, separated by comma, a pipe or space. The following are all valid all will output this way when --help is used.

"-p, --pepper" "-p|--pepper" "-p --pepper"

Examples:

// simple boolean defaulting to undefined
program.option('-p, --pepper', 'add pepper');

program.pepper
// => undefined

--pepper
program.pepper
// => true

// simple boolean defaulting to true (unless non-negated option is also defined)
program.option('-C, --no-cheese', 'remove cheese');

program.cheese
// => true

--no-cheese
program.cheese
// => false

// required argument
program.option('-C, --chdir <path>', 'change the working directory');

--chdir /tmp
program.chdir
// => "/tmp"

// optional argument
program.option('-c, --cheese [type]', 'add cheese [marble]');