#61 Add usemap option to image directive

[Leandro Lucarella]

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).

[Leandro Lucarella]

Patch against trunk r6021

[Leandro Lucarella]

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).

[Günter Milde]

As an "image map" is only supported by HTML output and a rarely used feature, the cost of increased complexity and documentation needs exceeds the benefits for Docutils.
Possible workarounds are raw HTML and embedded SVG images (cf. the solved [feature-requests:#100]).