Add parameter to show supported files for extraction

[nuest]

For users of extract tool, it would be nice to learn about the supported formats without going to the online documentation.

For software embedding the extract tool, it would be useful if this list comes in a parseable format, e.g. "new line delimited", so that it may be used in their own workflow.

The list could be file formats and file names, like this:

$ python o2rmeta.py extract -formats

*.r
*.rmd
*.shp
bagit.txt

I am unsure about the placeholders/regexes here, because within extract just the file extension is checked. The output could also be something like

    extensions: .r .rmd .shp
    files: bagit.txt

[ghost]

included with 1f627ae894602218c99a2ec8859c5d0430e17b98