Print "success" if error code is 0

[corkrean]

Presently, nothing is printed to stdout if a write is successful. We could enhance the usability of Zed by printing a success message after a write returns error code 0.

[josephschorr]

@corkrean We print out the zedtoken on success. Do you think we need more than that?

[jzelinskie]

Most unix-like tools don't output on success, so we tried that at first. However, if there's something that makes sense to output, we should (e.g. Joey's example of the zedtoken on write).