optparse.rb-getopts() : Add printing default value and comment

getopts() can receive default value, but it doesn't print in '--help' output.
I'd like to add printing default values. And also I add printing comment for options.
Please see the below example. 

```ruby
#!/bin/env ruby
require 'optparse'
opt = ARGV.getopts("",
                   "alpha:",
                   "beta:B",
                   "gamma:G;This is description"
                   )

# expected output with '--help' option are below
#
#  --alpha VAL
#  --beta  VAL(=B)
#  --gamma VAL(=G) # This is description
```

And if you don't mind, please merge it to official repos.
Thanks,