Use `OptionParser` to parse options

[st0012]

Currently IRB parses command line options with IRB.parse_opts, which means the output of irb --help needs to be manually updated. We should use OptionParser instead to make the task easier.

[jeremyevans]

I would be open to possible refactoring/method extraction so that you could override methods to get the behavior you want. However, understand that I am not the net-http maintainer, so you should get @nurse's opinion.

[nurse]

I once discussed about a hook for DNS resolution, but failed to get a consensus. I'll discussed about this topic again in a next developer meeting.

[stanhu]

I should note that Golang supports something similar: https://www.agwa.name/blog/post/preventing_server_side_request_forgery_in_golang

Reference: https://github.com/golang/go/issues/55301.