Snowfakery

`random_number` can now accept a "step" argument as described in the docs.

Allow a short-form random_choice syntax that matches the docs.

`date` now casts datetimes to dates instead of returning them unchanged.

The --target-number value now allows its two sub-arguments to be specified in either order: Tablename then number or number then tablename.

The command line interface gives better error messages about unknown
file extensions.

The continuation file format changed. Do not upgrade Snowfakery while
running a long-running Snowfakery process. The new configuration file
should be less prone to issues relating to failures to serialize results.

Improvements to plugin API: add `evaluate_raw` and `simplify`

Dependency updates.

Minor improvements in error reporting.

Fix a deployment problem relating to version.txt

Support for External Datasets. (PR 102)

Macros within Macros now work correctly. (PR 120)

It is now possible to have forward references to Nicknames. (PR 113)

Closed Issue 42: relating to relative month parsing

Closed Issue 131: Parse errors would result in an invalid rather than empty
JSON file.

Change the architecture to always initialize the database based on the recipe
and never do it from the CumulusCI Mapping file.

Update URLs for PyPI. No changes to the Snowfakery code itself.