Extended the rST parser's handling of roles.

Extended the rST parser's handling of roles to more closely match rST reference.

rST parser now supports:
    * All built-in rST roles
    * New role definition
    * Role inheritance

Issues/TODO:
    * Silently ignores illegal fields on roles
    * Silently drops class annotations for roles
    * Only supports :format: fields with a single format for :raw: roles, requires a change to Text.Pandoc.Definition.Format to support multiple formats.
    * Allows direct use of :raw: role, rST only allows indirect (i.e., inherited use of :raw:).