Remove distinction between offer/answer constraints in constructor

This issue was raised in a comment by @nazar-pc and didn't want it to get lost:

> I have a generic concern about constraints separation. Second argument `constraints` in `RTCPeerConnection` was removed from the spec a long time ago. At the same time answer/offer constraints are obvious from the context.
>
> I'd like to see a single `peer.setConstraints()` method that under the hood resolves to either `offerConstraints` or `answerConstraints` and do not even expose this separation for the user. In future major version I'd remove this separation entirely in constructor options too.

_Originally posted by @nazar-pc in https://github.com/feross/simple-peer/pull/324#issuecomment-396762783_