Add Increment and Decrement Functions

[erfan-goodarzi]

This PR Adds increments and decrements functions.

Resolves #77

[ASafaeirad]

I'm not sure we need to add the step argument 🤔
It becomes sum and subtract function. Also, people can provide a negative number as a step, which is counterintuitive.

[erfan-goodarzi]

I'm not sure we need to add the step argument 🤔 It becomes sum and subtract function. Also, people can provide a negative number as a step, which is counterintuitive.

The step is optional. Our function works without providing any step.
The thing is we can consider it like this: increase by 1, increase by 2, increase by 3. In any case inc function is a sum function.

if we decided to keep the step i'll handle the negative case that u mentioned before.