Make borders optional

The red and green borders get in the way of the website's design that its being used in. Please accept a boolean value to make them optional.

Also, just a heads up:
If I add this styles:
```
.ImageDiff__before,
.ImageDiff__after {
  border: 0!important
}
```
... in swipe mode the divider border get out of container.

Anyway, awesome component! Thanks! :)
  