Add Support for Source freshness test #19
Description
Add the support to test for source freshness. This is especially useful when the data pipeline reads data from a source and expects new data in it.
The user should be able to call TinyTimmy.source_freshness() and pass in the date time column and how recent you expect the data.
Support can be added to enable the user pass in a filter to limit the amount of data scanned (similar to how dbt does it). This way the test can be done more efficiently.