account for hierarchy in --keep #48
Description
For example with UPX, the executable is in something like upx-linux-v1.0.0/upx. If I use --keep "upx-linux.+/upx" or something like that, the extraction fails because when we go to write the upx file, the output subdirectory upx-linux-v1.0.0 doesn't exist. we can always adjust the regex to account for that, but I'd rather make it possible to just get a solitary file if we want it.