Specify python3 without a giving a path

[CADBOT]

In many systems - such as homebrew and Ubuntu - the python binary is called python3 instead of python. Currently, the only way to make python-shell use python3 on those systems is to specify the path. This is not ideal because python3 won't have the same path for all users. Can we add an additional option to specify python3? I'm imagining something like

const options = {
  python3: true // defaults to false
};

If approved, I can craft a PR for this myself.

[Almenon]

I ran into the same problem in AREPL, so this would definitely be nice to have.

This is a very easy change so I've taken the liberty of coding it (well, adapating it from my previous fix in AREPL):

if(options.pythonPath == null){
  // for non-windows OS it is best to use python3 instead of python
  // Mac and Ubuntu both have python being v2 by default
  // archlinux and freebsd both use v3 as default, but also provide python3 command
  pythonPath = process.platform != "win32" ? "python3" : "python"
}
else pythonPath = options.pythonPath

Looks good? If you want the open source cred feel free to test it and submit a PR, otherwise I can do it when I fix some of the other open issues.