Combinations of a set of lengths

I thought this addition might be handy
```
combinations(a, T::AbstractVector{<:Integer}) = Iterators.flatten([combinations(a, t) for t in T])
julia> collect(combinations(1:5, 2:3))
20-element Vector{Vector{Int64}}:
[1, 2]
[1, 3]
[1, 4]
[1, 5]
⋮
[2, 3, 4]
[2, 3, 5]
[2, 4, 5]
[3, 4, 5]
```