Introduce uniform interface for accessing loss function parameters

Perhaps I'm missing something, but this does not appear to exist at present. For most losses with a parameter, you access using property, as in

```julia
loss = HuberLoss(0.5)

julia> loss.d
0.5
```

But for an LPDistLoss there no field for `p`, only a type parameter:

```julia
julia> loss=LPDistLoss(2.5)
LPDistLoss{2.5}()

julia> propertynames(loss)
()
```

One suggestion would be overload `Base.getproperty`/`Base.propertynames` for cases of the second kind (I think these are just `LPDistLoss`, `L1DistLoss`, `L2DistLoss`, but I haven't checked lately), or to implement `StatsBase.params` or both.

I ran into this issue when wrapping the losses for use in MLJ and was obliged to write my own [interface](https://github.com/JuliaAI/MLJBase.jl/blob/a202a689ee3272ce7541ebeeb9a006cec733c4c9/src/measures/loss_functions_interface.jl#L26), which is not robust to extensions of LossFunctions.jl