adding close to Asciidoctor to avoid to leak when creating an asciidoctor instance

[rmannibucau]

Currently Asciidoctor instance can't be released.
With JRuby it means at least 2 pools are leaking each time an instance is created.
It leads to a lot of issues in several environments.
This PR adds a close() method to Asciidoctor and ensures JRuby instance is properly released when called.

Saved ~1mn on 1mn16s of execution with exec-maven-plugin for instance and avoids leakage in EE context.

[headius]

FWIW, a JRuby instance that becomes GCable will eventually shut itself down, but we strongly recommend tearing it down once the instance is no longer needed.

PR seems 👍 to me.

[robertpanzer]

Thanks!
Just have to remember that the next release has to be 2.2.0 instead of 2.1.1.