Add mode `concat`

[ComMouse]

Since the copy mode just copies files but compress mode do both concatenation and compression. In some scenes only concat (concatenation) is required.

Thx.

[arrowrowe]

开启 concat 而非 compress / copy 的使用场景是?

[ComMouse]

When some assets have been already compressed, but we still want to combine files into one..

[arrowrowe]

IMHO concat may become a special option for some assets, instead of a global mode like copy and compress.