Support symbolize_names option for getopts

[JunichiIto]

getopts method always returns string keys. If I need symbol keys, I have to convert them:

options = ARGV.getopts('l', 'w', 'c').transform_keys(&:to_sym)
#=> {:l => true, :w => false, :c => true}

It would be useful it can take symbolize_names option like JSON.parse.

options = ARGV.getopts('l', 'w', 'c', symbolize_names: true)
#=> {:l => true, :w => false, :c => true}

I submitted PR to resolve this: #46