Merge n slices in one

How do you feel about adding functionality lo.Merge to combine multiple slices? It's often necessary to write extra boilerplate to create a result slice and iterate over all slices.

```go
slice1 := []int{1,2,3}
slice2 := []int{10,15}

res := lo.Merge(slice1, slice2)

fmt.Printlnt(res) 
// Expecting result: [1,2,3,10,15]
```
