Add negative slicing support

[danielenricocahall]

For slicing (i.e; [1:-1]), add support for negative values by adding the length to the value. Basic idea:

test = "test"
test[1:-1] == "es" # True
test[1:len(test) - 1] == "es"  # True
test[1:len(test) - 1]  == test[1:-1] # True

[btwael]

Good job, thank you.