Adding r2b_tikz directive

Example:

    .. r2b_tikz::
    
       \fill [red] (2,1.5) rectangle (8,2.5);
       \fill [blue] (1,1) rectangle (3,3);
       \fill [blue] (4,1) rectangle (6,3);
       \fill [blue] (7,1) rectangle (9,3);