Add tests

Some extremely rudimentary tests that cover several quantitative use cases:

* Dates tracks differences between hours, dates with words, and abbreviated dates
* Errors deals with CSVs that have "broken" as a result of an incorrect change. In these cases the differ should fall back on line diffs, the git default
* Numbers deals with whole numbers, as well as quantities
* Strings deals with words, and should probably rely on the git word matching default

/cc @jonmagic