Allow bringing your own mruby build

Hi there,

the mruby version being shipped here hasn't seen updates in several years, the ecosystem has changed since then and security issues have also been fixed.

I am proposing that one should have the option to dynamically link against a .so or .dll build of mruby or maybe even add a RPC API so one can script H2O with any language and not just mruby.

Thoughts?