improve create target --seed

[kjk]

1. It should also accept .zip files as it's more widely supported than tar.gz

2. It should validate that the file given is in valid format (.tar.gz or .zip) before uploading it

[kjk]

Also validate the directory structure inside the archive is correct.

Can I just archive top-level files (say, foo.html, bar.html) or do they have to be in a directory (like corpus/foo.html and corpus/bar.html)?

Is the directory name significant?

[yevgenypats]

Actually it must be top-level files without a directory inside. Agree that some sort of validation is needed. Also agree on the .zip though it will need some tweaks on the backend so let's leave this Issue open for now.