Inference preferable format based on the situation

[StoneDot]

This proposal is related to #176.

As I mentioned in #176, the dynein format is preferable when showing output for human reading, but JSON is more suitable for reading by programs. I propose inferring the preferred format using atty.

If the user uses a pipe, a program consumes output; if the user does not, it means that a human checks the output. I think this correspondence can be used to infer a preferable format.