Make attribute option flag configurable

The built-in exporters (pdf, epub, html-single) all assume that the command being run recognizes the -a option flag as a document attribute assignment. While this is true for Asciidoctor-based converters, this may not be true for all converters.

The name of the option flag should be configurable in the build section of the Assembler configuration file. The proposed key name is attribute_option_flag. For example:

build:
  command: my-asciidoc-converter
  attribute_option_flag: '--attribute'

The <name>=<value> will be added after the flag each time it is used.