Allow String#%

I like to use:

```
"This %s a string, with %d placeholders. %s with it" % ["is", 3, "DEAL"]
```

IMHO this is a reasonable alternative to:

```
"This #{"is"} a string, with #{3} placeholders. #{"DEAL"} with it"
```

So this change allows for it.
Merge pull request #1 from pjaspers/patch-1

Allow String#%