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.
Made patch more readable thanks to @Ortuna's comment https://github.com/padrino/padrino-framework/pull/1452/files#r6968956
Merge pull request #1452 from padrino/padrino-start-accepts-handler-options

Allow padrino start to take handler specific options