7z should have Archive method

[drwpeng]

7z should implement Archive or other methods, which makes the Identity method unable to determine the file type

[mholt]

That requires that a 7z writer be implemented in pure Go first, which I haven't found.

[drwpeng]

It seems that there is no pure Go implementation at present, but you can use the empty method like rar

[mholt]

Oh, right! That's true, I guess we should do that at least.