Add configurable startup logging as a block

[twisterghost]

Allow kemal.run to optionally take a code block to change the post-start logging, or run any other post-start code at that time. This fixes part of #290, wasn't sure how to handle the shut-down text.

I hope I'm not missing some glaringly obvious configuration here that already allows for this!

[samueleaton]

Oh this is great.

specs?

[twisterghost]

I'll look into writing some specs for it. I didn't immediately see a file for them to live in. If you have a suggestion, let me know.

[sam0x17]

I don't understand how to use this to override the default startup message. Passing a custom block seems to still result in the "Kemal is ready to lead!" message. I'll happily document this if I can get some clarification on how to actually do it.

[sam0x17]

note: for those looking its:

def Kemal.display_startup_message(config, server)
  addresses = server.addresses.map { |address| "#{config.scheme}://#{address}" }.join ", "
  log "[#{config.env}] my cool server is live at #{addresses}"
end

Just make sure that is placed after the require "kemal" line