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).
Merge pull request #16 from b3/fileglob-extension

Updated FileGlob plugin to populate file name parts.