Support for properties (e.g. isconvex)

While not the most fun task, this could be a good next step.

Something like the following:

```julia
isdifferentiable(::L1Penalty) = false
isdifferentiable(::L1Penalty, at) = at != 0
isconvex(::L1Penalty) = true
isstrictlyconvex(::L1Penalty) = false
isstronglyconvex(::L1Penalty) = false
```