Yaml-to-sqlite

- New feature: `yaml-to-sqlite dogs.db dogs.yaml --single-column=name` can be used to import YAML files consisting of a list of items into a single table with a single primary key column. [5](https://github.com/simonw/yaml-to-sqlite/issues/5)
- Importing YAML files with new columns will now alter the table to add those columns. [4](https://github.com/simonw/yaml-to-sqlite/issues/4)

- Updated for latest `sqlite-utils` library
- Fixed bug where import failed if user did not provide a `--pk` option

* Now handles nested objects that contain dates, 1

* Better help for `--pk`

Better README