Drop Julia 0.6 and more

• Fix open_url on Windows (@traversaro): #73
• Update MeshCat javascript for better lighting: #70
• Require Julia 0.7 or higher: #69
• Respect mesh color (@ryanelandt): #62
• Fix material handling for PointClouds: #59
• Documentation and error messages (@tkoolen, @blegat): #65 #64