Improved TypeScript support

[speedytwenty]

There are presently no tests that confirm the CliTable3 types defined in index.d.ts are accurate and functional. There have been mentions of issues with using cli-table3 with TypeScipt (#55).

This PR will add tests that verify strict TypeScript support and resolve any existing issues with the current types.

It's unclear whether this will introduce a "breaking change" as it seems the TypeScript support is likely already broken for strict type checking.