Let modifiers of -i (+d|l|o|s) available for -o.

But for some modules it is not possible to use -i (at least in direct way). 

For example, I will have to do this:

```
gmt pshistogram @v3206_06.txt -F -T50 -I -o3 | gmt info -i+d100 -C -o0
5.26
```

Instead of 
```
gmt pshistogram @v3206_06.txt -F -T50 -I -o3+d100
pshistogram [ERROR]: The -o option does not take +d|l|o|s modifiers; consider -i instead.

```


