add options to the constructor

[marten-seemann]

I was planning to use an Option this to set a mock dialer / listener (in a follow-up PR), as suggested in #95 (review).
However, this change turns out to be more problematic than I thought, since the reflection magic in go-libp2p doesn't like the Option parameter here. @Stebalien, is there a nice way to make this work with the reflections?

[Stebalien]

Well... switching to a real dependency injection system would fix this. But we should also be able to fix it in the short-term in go-libp2p by ignoring variadic arguments in the reflection magic.

[Stebalien]

libp2p/go-libp2p#1204