Add diff percentage to output

[eWert-Online]

This adds the diff in percentage to the output.
The format now is "Different pixels: 7586 (1.137331%)"

Fixes #8

[dmtrKovalenko]

Thanks! This is a nice addition, we need to also think about how we will return this information to the parent process (e.g. we have bindings to nodejs)

[eWert-Online]

Ah you are right. I didnt see that there were node bindings.

As far as I can see, the difference in pixels is also not returned to node, right?
So we would need to think about a general solution for returning both pixels and the percentage to node.

I will see if I can think of something later today :)

[dmtrKovalenko]

Thanks! I think this PR is good to go, I will merge this. For the return perspective, I think we need to just print the output to stdout and then parse in node.

So basically print something like 14254;2.456 and in node take this string and split it by ";" and use values. I'd like to avoid any buffers, streams, or IPC but if you have any idea – Id like to hear! :)