Updating `subtex.py` script.

Updating `--strip-si` option to handle custom figure macros.  This
entailed overhauling `parse_table_and_figure_refs` and
`get_top_level_contents` functions, and the addition of
`top_level_content_iter` and `parse_custom_figure` functions.
Merge branch 'master' of github.com:joaks1/subtex

Merging remote changes to README that I forgot to pull before
working on `subtex.py` script.