Handle empty matches for operations change and delete

When an empty match is found, it can't be selected as discussed in #1.

I used a small trick to make it work for the operators `c` and `d`. The mappings insert a temporary character `x` in the empty match and visually select that character. The pending change or delete command will remove that character immediately. This works now:
```
a,b,,c,d
```
With cursor on `a` executing `cin,f<Esc>..` leads to:
```
a,f,f,f,d
```

Close #1.