A data structure used for storing and validating [decision tables][1].

Features:
 * each column has its own set of (string) values
 * add and remove rows
 * check that the table is complete (that is, has every possible input row)

Roadmap to v0.2:
 * arbitrary output types
 * "don't care" cells

Future:
 * Range cells
 * Multiple input types
 * CLI
 * Find missing inputs

[1]: https://www.hillelwayne.com/post/decision-tables/