Plotting using Plot.jl and recipes

non-exhaustive list of things that would be interesting to plot:

```
plot(w::Function,dom::Tuple(Float64,Float64))
plot(m::Measure)
histogram(x::Vector{Float64},op::Union{OrthoPolyQ, MultiOrthoPoly})
histogram(op::Union{OrthoPolyQ, MultiOrthoPoly)
```

This should be done with `Plots.jl` and recipes.

caveat: when plotting the `w` we need to consider cases of infinite support...