foamDictionary: Added '-diff <dict>' option

  - Write differences with respect to the specified dictionary
    (or sub entry if -entry specified)

  - Write the differences with respect to a template dictionary:
      foamDictionary 0/U -diff $FOAM_ETC/templates/closedVolume/0/U

  - Write the differences in boundaryField with respect to a
    template dictionary:
      foamDictionary 0/U -diff $FOAM_ETC/templates/closedVolume/0/U \
        -entry boundaryField

Patch contributed by Mattijs Janssens