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.

https://github.com/cujojs/most/blob/bfed148f1bfd8bc06a972a2cb17599371d8da52f/src/index.js.flow#L50-L55

Todo

• [ ] ~Unit tests for new or changed APIs and/or functionality~
• [x] Documentation, including examples