Changes to use CLI args to configure VIEWER_ORIGIN for lighthouse server

[bkjam]

Hi,

I like the solution in #637 which allows us to change the default self-hosted lighthouse report viewer. However, I noticed that the $VIEWER_ORIGIN environment variable works only if you build a custom @lhci/server package. Hence, I made some modifications to configure $VIEWER_ORIGIN for @lhci/server via the CLI args.

I changed the following things:

• added an optional --viewer.origin cli args for server (Default: https://googlechrome.github.io)
• added /v1/viewer/origin endpoint that retrieves the viewer.origin value
• updated the server docs

This may not be the most optimal solution, so I would appreciate any feedback on my approach and code changes. Thank you :)

[connorjclark]

However, I noticed that the $VIEWER_ORIGIN environment variable works only if you build a custom @lhci/server package.

Do you mean that it only works if you host your own LHCI server? As I understand, you don't need to build anything yourself - the package from npm for @lhci/server will work if you run it with VIEWER_ORIGIN env defined.

If you aren't hosting your own LHCI server, I'm a bit confused why you'd want to be hosting your own report viewer. If that's the case can you explain your use case a bit more?

Otherwise, the code looks good, I'm just not sure why its needed yet.