Make subscribe callbacks optional in typings

[vlinder]

Summary

Update docs and typescript typings to reflect that methods on the Observer all are optional.

Here is what is linked in the docs regarding the Observer: https://github.com/tc39/proposal-observable#observer

Flow types seem to be correct already.

most/src/index.js.flow

Lines 50 to 55 in bfed148

	export type Subscriber<A> = {
	+next?: (value: A) => void;
	+error?: (err: Error) => void;
	// complete value parameter is deprecated
	+complete?: (value?: A) => void;
	}

Todo

• Unit tests for new or changed APIs and/or functionality
• Documentation, including examples