Change. Allows `exattrib` field be array of flag (See #3)
Add. Flags to make `exattrib` on Unix/Fat32.

Still it is very verbose.
```Lua
-- make attr for `-rw-r-----` on Unix
local desc = {
    platform = 'unix',
    exattrib = {
      ZipWriter.NIX_FILE_ATTR.IFREG,
      ZipWriter.NIX_FILE_ATTR.IRUSR,
      ZipWriter.NIX_FILE_ATTR.IWUSR,
      ZipWriter.NIX_FILE_ATTR.IRGRP,
      ZipWriter.DOS_FILE_ATTR.ARCH,
    },
}
```
Also because NIX_FILE_ATTR `lshifted` to 16 you can not use directly value from `chmod` function.
Update Travis file.