Refs #58

Fixes remaining instances of this bug

Refs #20, #55, #57

* Find data files in subdirectories in archives

See #54 for discussion. This also ignores files in the new "assets"
directory, which appear to be some stuff for a browser interface
Twitter's created.

* Fix list-member importer

It appears in list data that some rows contain a `urls` key with a list
of URLs, while others contain a `url` key with just a single one. This
change supports either way.

* Fix tweet import

This was working, sorta, but wasn't properly unpacking the tweet
data into columns. This commit fixes that in what I think should
be a backwards-compatible way.

Thanks, @mikepqr

Refs #41

Also fixed bug from the sqlite-utils 2.x upgrade caused
by checking db[table].exists instead of db[table].exists()

Closes #38

Refs #35, refs #8. Still needs documentation.

By bumping sqlite-utils dependency to get this fix:
https://github.com/simonw/sqlite-utils/issues/52

Thanks to Jacob Kaplan-Moss: https://twitter.com/jacobian/status/1192510111719313408