Ticket #39 (new defect)

Description

There are two kinds of figures that we need to support. One is just added by the user and refers to a preexisting file. Another is one that is grabbed from the currently executing Python code (e.g. grab_figure()). The latter should be an element under the appropriate cell.

<ipython-log>
  <cell number="1">
    <input>plot(range(10))</input>
    <figure source="matplotlib.grab_png" filename="figure_1.png">
      <number>2</number>
      <caption>This is a nifty plot</caption>
      <dpi>72</dpi>
    </figure>
  </cell>
</ipython-log>

The source attribute is an identifier for which the shell will search in a registry of such backends.

The filename attribute is the filename of the cached output.

The subelements correspond to keyword arguments of the function identified by source. Such a function should expect to receive arguments in the form of strings.