Improve unblob "skip-extraction" mode of operation

[qkaiser]

The following improvements have been made in order to let user simply "scan" a file:

1. do not create the root level extraction dir when executed with the --skip-extraction option.
2. display a summary of identified chunks when executed with the --skip-extraction option.

This is related to #624.

Right now the information displayed to the user is fairly limited:

But the introduction of metadata reporting in chunks (see #557) would allow us to display detailed information in the "description" column, such as endianness, version, compression, block size, creation size, etc.

[aserper]

Makes sense and looks good! 👍

[e3krisztian]

I thinks this is fine as is (only code reordering proposed), however there is no test for skip_extraction at all, so future changes could inadvertently break the newly added functionality.

[qkaiser]

@e3krisztian I applied your recommendations, introduced tests for both the cli and processing parts of skip_extraction option and fixed a typo in the CLI (skip_extraction should have been skip-extraction from the beginning).

[e3krisztian]

Cool! :)