TypeScript typings additions and fixes (#307)
TypeScript typings additions and fixes (#307)

* Modify CommandGeneratorFunction return types

Add a Promise<string> return type to CommandGeneratorFunction.
This enables generator functions that use the ES7-style async/await
syntax to specify the required Promise return type for async functions.

* Add Promise<void> return type

* Fix incorrect registerCommand return type

abalabahaha/eris,briantanner/eris,Flyy-y/eris