Removed some cruft.
Implemented Ext_implicit_figures.

* In markdown reader, add a '\1' character to the beginning
  of the title of an image that is alone in its paragraph,
  if implicit_figures extension is selected.
* In writers, check for Para [Image alt (src,'\1':tit)] and treat
  it as a figure if possible.
* Updated tests.

This is a bit of a hack, but it allows us to make implicit_figures
an extension of the markdown reader, rather than the writers.