[Feature request] Change extract syntax for multi-slice/file archives

Some games' offline gog installers come split into several files.

Currently there is no usable way to make sure all files are present, and leaving the files in-place while extracting.

I suggest the following change to the schema:

• the value of file in extract may be a file_id or a list of file_ids
  • the yaml is invalid, if file is a list of strings and format is unspecified or not equal to "gog"

And the following changes in behavior:

• if file is not a list of strings: unchanged
• else, and if the and format is gog:
  • Assert that each file is present
  • Assert that all files are contained in the same folder
  • run innoextract using the first entry in file as arg