Fix open_url on Windows

[traversaro]

In Julia 1.0.2 run seems not be be working with command prompt builtin commands.

In particular I get this error:

julia> vis = Visualizer()
MeshCat Visualizer with path /meshcat

julia> open(vis)
[ Info: Serving MeshCat visualizer at http://127.0.0.1:8700
[ Info: Listening on: Sockets.InetAddr{Sockets.IPv4}(ip"127.0.0.1", 0x21fc)
Could not open browser automatically: Base.IOError("could not spawn `start http://127.0.0.1:8700`: n
o such file or directory (ENOENT)", -4058)
Please open the following URL in your browser:
http://127.0.0.1:8700

Explicitly launching the command prompt executable cmd.exe that executes the builtin start command with the /C option seems to be working fine instead. See JuliaLang/julia#23597 for a related discussion.

[traversaro]

cc @prajval10

[rdeits]

Thanks!