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
steak generator written, tests a work in progress.
remove untested misplaced special treatment of Ohm
cleanup error generators
do not mutate provided options
extract select option tags building
fix html_safe in labeled group
do not touch options, allow button_to with block and no caption
remove old syntax with sticking boolean to options array/hash
Merge pull request #1452 from padrino/padrino-start-accepts-handler-options

Allow padrino start to take handler specific options
refactor render partial
do not sort dependencies, fixes #1458
restore untested $LOAD_PATH behavior, add tests, fixes #1463
cache content_type, fixes #1462
log 404 error of unmounted path, fixes #1459
Merge pull request #1455 from namusyaka/add-steak

Add steak support
Change request.accepts to an empty array if request.accepts is `*/*`.
provide more examples for the instantiation of the cache stores
Merge pull request #1467 from minad/cache-examples

provide more examples for the instantiation of the cache stores