Custom decorator

This PR introduces new style decorator, `{r, g, b}`.
The tuple can be used as same as existing decorators.
Each element `r`, `g` and `b` should be in `0..5`.

```ex
Colorful.string("hello", {2, 3, 4})
Colorful.string("hello", [:underline, {2, 3, 4}])
```