Db-to-sqlite

Fixed compatibility with `sqlite-utils` 2.x - 20

Fix for 18 - no longer throws error on empty tables

* Improvements to README

See the [README](https://github.com/simonw/db-to-sqlite/blob/1.0/README.md) for full usage instructions.

* Instead of using `--connection` the connection string is now a required positional argument, 14
* `--sql` must now be accompanied by `--output` specifying the table the query results should be written to
* `--redact tablename columnname` option can be used to redact values, 2
* Foreign keys are now created with indexes, use `--no-index-fks` to disable this, 12
* `--table` can now be used multiple times, 6
* README and `--help` now include example connection strings
* README also details how this can be used with Heroku Postgres

* Added `--progress` option to show progress bars during import - 7

- Support `pip install db-to-sqlite[postgresql]` 4
- Documentation for both that and `pip install db-to-sqlite[mysql]`