Plots.jl recipes

[ChrisRackauckas]

It would be nice if the types here had Plots.jl recipes so that way the plots could match the outputs of a lot of other packages. Right now this seems confined to Gadfly.

[bdeonovic]

See #144

[goedman]

To convert to an MCMCChains.Chains object:

chn2 = MCMCChains.Chains(chn.value, String.(chn.names))

where chn is the Mamba Chains object, e.g. see m4.4.

(Mamba.jl on Julia v1.1, it doesn't run on v1.2 on my system). MCMCChains.Chains are not a 100% replacement for Mamba.Chains (yet), but pretty close.