Customer Side: Add To Cart

**Add To Cart** option include:
1. option for adding an item in the cart, i.e "+" button
2. option for subtracting the quantity of an item in the cart, i.e. "-" button
3. if(quanity_of_an_item==1): Upon -ve button press {quantity_of_an_item=0}
4. elif(quantity_of_an_item==0):Upon -ve button press {pop-message: Quantity Already "0"/or some other message that tells the same}