CLI flag to choose whether to open a new lighthouse page or not

**Feature request summary**

A CLI flag (something like `--reuse-page`) to re-use the existing page that was running on the browser that puppeteer connects to.
The page can be selected from `await lhBrowser.pages()`.

**Are you willing to work on this yourself?**
Yes

**What is the motivation or use case for changing this?**
It will help run lighthouse programmatically inside applications like CEF and Electron where opening a new page (by default) causes a failure when connecting to it through remote debugging port.

**How is this beneficial to Lighthouse?**
It will widen the audience to which lighthouse can be used and increase the tool's users.

Mentioned the changes in the screenshot: