resolves #11 fix compatibility with Asciidoctor 1.5.x

The following changes are necessary to make asciidoc-bib compatible with Asciidoctor 1.5.x.

* Change Preprocessor#process method arguments to `document, reader`
* Read lines from reader in Preprocessor#process method
* Push lines back onto reader after processing and return reader in Preprocessor#process method
* Remove argument to Extensions.register block

The extension API was not considered stable prior to Asciidoctor 1.5.x, so I wouldn't recommend supporting older versions.