Migrate to TypeScript 2.1

[kitsonk]

There are several changes/improvements with TypeScript 2.1 which we will want to adopt (plus likely to be bugs).

The important changes are:

• async/await for ES5 emits
• Static types of dynamically named properties
• Mapped types (which will allow partial types)
• ES8 object spread/rest
• Better inference for literal types and also functional boundary type narrowing issues and other improvements to CFA
• String and number literal narrowing on strict equality
• Configuration inheritance

We should make any issues related to 2.1 as part of this epic.