Implement attributes-block parser

New parse_attributes_block() function and supporting functions:

* parse_attributes_block_identifier()
* parse_attributes_block_classes()
* parse_attributes_block_attributes()
Correct attributes regex and add more chunk tests

Make regular expression to extract attributes from attributes block
non-greedy.

Add code to raise ValueError in the event of duplicate attributes.

Add more tests to cover individual functions, empty attributes and
duplicates.