Add conduitParserEither variation that returns Either instead of throwing an exception

Hi Michael,

I've made a few improvements to attoparsec-conduit that I think would be good to have in the library. Let me know what you think. I kept your parsing logic-workflow exactly the same and just added the new functionality as an option. There is a bit of duplication in there and we may be able have one function use the other, but I've kept it simple for now.

Also added a test case, which I think would be good to have.

My first commit has some explanation as to why.

Cheers,
Oz