add plotting for cartpole and mountaincar with Plots.jl

[jamblejoe]

see #246

[jamblejoe]

The cartpole and the mountain car look as follows. I am happy to change the appearance when necessary.

[zsunberg]

I wonder if Plots.jl is the right package to do this with since it is not really a data plot. Maybe something like Compose.jl or Cairo.jl that is focused on general 2D graphics would be better (A compose mountaincar example is here: https://github.com/JuliaPOMDP/QuickPOMDPs.jl/blob/d5c51e68e1a089b1e0fe9518493b27e31cd4c1aa/examples/mountaincar_with_visualization.jl#L18-L27)

[jamblejoe]

@zsunberg I agree. The previous implementation used GR but did not return a figure object to quickly do animations and it did not work out of the box with jupyter notebooks.

The PR was a fix I initially wrote for myself but was asked to create a PR. I am only familiar with Plots.jl, which is using GR as backend by default. It gets the job - animations and jupyter support - done quick and dirty.

I dont know Compose.jl or Cairo.jl.

[findmyway]

I prefer to merge this first.
Rendering is not a core component in RLEnvs.jl and this PR moves one step further to drop the dependency of GR.
Different environments may have many different approaches to render environments. For example, GridWorlds.jl uses Makie, the Atari simply returns a Matrix. At least this PR gives users an option to plot the default CartPole and MountainCar environment.

[findmyway]

@all-contributors please add @jamblejoe for code

[allcontributors]

@findmyway

I've put up a pull request to add @jamblejoe! 🎉