* 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.
Previously header ids were autogenerated by the writers.
Now they are generated (unless supplied explicitly) in the
markdown parser, if the `header_identifiers` extension is
selected.
In addition, the textile reader now supports id attributes on
headers.
