Support loading with user settings

[almahdi404]

It would be nice if there was an option to load it with custom user settings.

vscode-test-web --settings=settings.json

settings.json

{
  "workbench.colorTheme": "Visual Studio Dark"
}

[yiliang114]

I 'd like to try this feature. Does the official team think this feature can be incorporated? @aeschli