handle visual-block mode

We would need to check that regions of matched lines are vertically aligned to
make this work intuitively.

E.g.
```
  This region highlighted
  ---
abcdefg
hijklmn
  ---

     ...should match this region
     --
.....cde..
.....jkl..
     --


     ...But not this region
     ---
.....cde..
...jkl..
   ---

```
