Add API for Statement

[Benjamin-Davies]

This library supplies a promisified of node-sqlite3's Database class. However, it does not do anything about the Statement class.

The Statement class supplies an API that allows the user to supply parameters to their queries. This is better than the API that is supplied by the Database class as it allows the user to avoid SQL injection vulnerabilities (TryGhost/node-sqlite3/issues/57). By not extending this class, your library is discouraging its use and encouraging bad practices.