* Sets default flag to `false` for `showSubtraction` flag.
* Removes extra span for else case that was causing some tests to
fail. Moreover, the previous behavior was defined as that.
* Adds custom test for the showSubtraction option, both using the
`showAdded` and `showRemoved` flags.
Adds "Show Subtraction" button to allow seeing only the difference
between two texts.
When selected and combined, user can see only the characters or
words that were added. If not combined, with either removed or added
but selected, then nothing is displayed.
