Allow & include empty partitions among sets from `partitions(::Vector, ::Int)`

For some applications, it can be helpful to include in the set of partitions of a vector, the option of taking nothing, i.e., to include empty-partitions. Currently, however, `partitions(s::AbstractVector, m::Int)` will only consider non-empty partitions. I think it would be nice to allow the former as well, enabled e.g., by a keyword argument (e.g., `allow_empty`).

By example, this would turn the two following examples from:
```jl
julia> partitions([1,2,3], 2) |> collect
[[1, 2], [3]]
[[1, 3], [2]]
[[1], [2, 3]]

julia> partitions([1,2,3,4], 3) |> collect
[[1, 2], [3], [4]]
[[1, 3], [2], [4]]
[[1], [2, 3], [4]]
[[1, 4], [2], [3]]
[[1], [2, 4], [3]]
[[1], [2], [3, 4]]
```
to:
```jl
julia> partitions([1,2,3], 2; allow_empty=true) |> collect
[[1, 2], [3]]
[[1, 3], [2]]
[[1], [2, 3]]
[[1, 2, 3], []] # additional permutation

julia> partitions([1,2,3,4], 3; allow_empty=true) |> collect
[[1, 2], [3], [4]]
[[1, 3], [2], [4]]
[[1], [2, 3], [4]]
[[1, 4], [2], [3]]
[[1], [2, 4], [3]]
[[1], [2], [3, 4]]
[[1, 2, 3], [], [4]] # additional permutation
[[1, 2, 4], [3], []] # additional permutation
[[1, 2], [3, 4], []] # additional permutation
...
```

NB: This can currently be "simulated" by appending some token element, e.g., `0`, onto the input vector `m-1` times, and then considering token-elements as empty. I.e., `partitions(vcat(s, fill(0, m-1)), m)`. But this seems inelegant and generates redundant possibilities that need to be filtered out subsequently.