Allow to override the 'includeToc' setting on project-level

For the ASCIIDOCTOR formatter projects can now decide on their own
whether a Table Of Contents should be included at the beginning of
each document.

Change-Id: I6bd1e5556ad153b4763e54a052c8e9f96bc72e7f
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>