add ALGORITHM variable to /etc/default/zramswap

I suggest to allow users to set the compression algorithm in the default config, like this:

(this code, exept for the ALGORITHM part, is copied from the original script, zramswap)

    for core in $(seq 0 $(($cores - 1))); do
        echo $ALGORITHM > /sys/block/zram$core/comp_algorithm
        echo $(($ALLOCATION / $cores)) > /sys/block/zram$core/disksize
        mkswap /dev/zram$core
        swapon -p $PRIORITY /dev/zram$core
    done

Default algorithm should be set at the beginning, I suggest lz4. Available (on my system) is:

$ cat /sys/block/zram0/comp_algorithm
lzo lz4 [lz4hc]