Update Example: Let's make a giant string

* Add another function `add_bytes_with_plus` actually illustrating
  quadratic behavior for the `+=` operator.
* Add explaination for linear behavior due to `+=` optimizations
  in case of strings.
* Change the order of examples (move string interning example just
  before the giant string example).

Closes https://github.com/satwikkansal/wtfpython/issues/38