Use srings to describe exchange region comparison

... in preparation for things to come.
Add "expand" functionality

When one exchange region is completely contained within another,
replace the larger one with the smaller one.

The term "expand" may seem counter-intuitive, as the result is less
text, but think of it this way: the smaller region is "expanding"
to occupy the space previously defined by the larger region.
Clearly explain Visual mode mapping in README