Updated FileGlob plugin to populate file name parts.

For each matched files we populate three variables corresponding to the
three *natural* parts of a filename: dirname, basename and extension (being
everything after the last period). One use case is when we have a gallery with
different version of the same file (`.svg`, `.pdf`, `.eps`, `.png`, etc.).

I also updated some documentation about hidden, nice (untested ;-) FileGlob
features (when matched files are templer input files).