Make RAW formatter configurable

It can now be configured which formatter is used when the 'raw' URL
parameter is appended to the project documentation URL:

  [formatter "RAW"]
    formatter = PLAIN_TEXT

Change-Id: I97db4d2bed5a4a3d8b8f954fe1a46fee86e3001d
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>