Fix current position in visual mode.

Adjusts current position in visual mode to point to the last selected
character, not the next one.
This fixes issues like:

```
          cursror
          |
 text worda wordb
          ~
          ^selection
```

Before the fix if `iw` is pressed both words will be selected:
```
                cursror
                |
 text worda wordb
      ~~~~~~~~~~~
      ^selection
```
With this fix:
```
          cursror
          |
 text worda wordb
      ~~~~~
      ^selection
```
This also applies to other motions that depend on the current position.
Merge pull request #236 from igrekster/visual-position-fix

Fix current position in visual mode.