Feature suggestion: do not use concurrency when buffer count is 0

[mgsloan]

I think it'd be rather cool if this used runConduit directly when the buffer count is 0. This is handy when you want to use some stm-conduit code as a building block where some use-cases benefit from buffering while others do not.

It's not a big deal - quite straightforward to check if the buffer count is 0 and do this manually. But figured I may as well mention it anyway.