Add -u option for one unit size - issue/#39

[sumomo-99]

This pull request add a -u option to allow setting one unit size to like df -k.
usage:

$ sudo ./ps_mem.py -u k
Private  +   Shared  =  RAM used       Program
......
22180 KiB +    80 KiB = 22260 KiB       docker-containerd
37748 KiB +   568 KiB = 38316 KiB       netdata
61436 KiB +   265 KiB = 61701 KiB       dockerd
---------------------------------
                        197820 KiB
=================================

option
-u , --unit=U    Unit size. U are b,k,m,g,t (powers of 1024). Default unit is KiB.
b: byte
k: KiB
m: MiB
g: GiB
t: TiB

[pixelb]

Note this will mess up alignment
Also aligment may change over time
Note the --total output is unformatted and so useful for processing/monitoring
Can you describe your use case in detail.

thanks

[sumomo-99]

One unit size is useful when I want to find out what process is large private or shared memory, in all processes.
If there is a sorting option by private or shared memory, this pull request may not be necessary.

thanks