Enhanced Pandoc's support for rST roles.

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:).
Clarified field values in RstCustomRoles.
Merge pull request #1163 from merijn/master

Extended the rST parser's handling of roles.