Add single argument (path) constructor

Also, document the constructor options made in this and the previous pull request.

I'm going to check on making that array constructor thing, just because it might be more efficient, and a bit easier than having to install the multiple io handles into whatever event loop or async method one is using. This part was just easier to bang out quick.

Thanks for being on-the-ball about merging these!

Would you mind if I opened some tickets on rt.cpan.org for discussing some other things? There are some cleanup tasks I'd like to do (tabs/spaces inconsistencies, use modern Test::More for done_testing and subtest, reorganize t/ and create xt/ for the critic stuff, etc...), but I don't want to just come in here and throw my weight around without at least discussing it with the authors.