decompress and plugins are inconsistent about whether you can pass in a stream

[glasser]

Most decompress plugins allow input to be a stream. But decompress itself requires it to be a Buffer or a filename, and the filename is converted to a Buffer not a stream. I get that eg decompress-tar takes a stream because it might get one from decompress-targz, but it's unclear why decompress-targz (eg) is allowed to take a stream.

Moreover, it would be beneficial from a performance standpoint if you could pass in a stream to plugins like the tar ones that support it. Yes, it would mean that automatic type detection wouldn't work, but if you're explicitly specifying a single plugin like decompressTargz then that doesn't matter.

#57 at least will document explicitly that this package always reads the whole file into memory. But it would be nice if this package (with such a nice API) didn't waste so much memory.