The option is needed to easily plug an image map to a reST image. This is
useful even when no 'imagemap' directive is implemented yet, because the
image map can be written by hand using a 'raw' directive and for plug-ins
that generate their own images using an image map (aafigure, for
instance).
Patch against trunk r6021
Arrr... aafigure is a bad example, because it can't do image maps (I picked that because is a popular one, but there are other examples that can use this, like graphviz and mscgen, which support image maps).