Add support for parsing child blocks

[robertpanzer]

From http://discuss.asciidoctor.org/AsciidoctorJ-Block-Processor-Problems-tp3150p3297.html

Second, we need to provide a way to parse child blocks. We talked about adding this feature to the createBlock method. I also think it's reasonable to add a method to AbstractBlock named either parseChildren() or parse(List<String>). In the first case, it would parse child blocks from the source lines passed to the block constructor. In the second case, it would parse blocks from an arbitrary set of lines and append them to the block as child nodes. I'm leaning more towards the second option.

[robertpanzer]

Fixed with #337