Adds a create method with classloader and gempath as parameters

Adds a create method with classloader and gempath as parameters resolving issue #258 .
I think we should use the existing `EnvironmentInjector#inject` (or overloaded method) to handle this logic because there is more to it than just setting the `GEM_PATH`. If the value is empty or null, the environment variable needs to be removed. See https://github.com/asciidoctor/asciidoctorj/blob/master/asciidoctorj-core/src/main/java/org/asciidoctor/internal/EnvironmentInjector.java#L17