Allow padrino start to take handler specific options

Modified `padrino start` to take an extra `--options` (`-O`) parameter to pass options to the rack handler.

For instance, with `puma` you can now do:
`bundle exec padrino s -O Threads=3:16` and the server will start with 3 to 16 threads.

This implementation follows the same syntax that [rackup](https://github.com/rack/rack/blob/master/lib/rack/server.rb\#L100) uses for compatibility.

Also implemented --server_options to print out the supported options for the selected handler (application server).

This has been requested a couple of times, the latest in #1451.

I'd still recommend people to run their applications through the application server directly -e.g., puma, unicorn, thin...- instead of using this but it might come in handy for development.

Thoughts @padrino/core-members?