Chrome extension: follow the inspected tab and update the docs

[erkamyaman]

The extension panel already connects live on localhost and 127.0.0.1: panel-bridge.js finds the devframe connection and passes baseURL to the SPA. What's left:

• Follow the inspected tab. With several tabs open, the panel shows whichever page reported last. Read the page id (sessionStorage['ng-devtools-page-id']) through chrome.devtools.inspectedWindow.eval, pass it to the SPA and filter to that page. Do it again on navigation.
• Select from the Elements panel. When the user selects an element ($0) in Chrome's Elements panel, select its component in the Components tab.
• More local hosts. Support *.localhost and [::1], and let users opt in to other dev hosts (LAN IP, tunnels) through optional host permissions.
• Clear status. When no devframe is found, say which URLs were tried and link to the setup docs, instead of opening the static view without explanation.
• Docs. The README's extension section still says the panel loads in static mode by default, and walks through an old setup guide. Replace it with how the current extension works and how to load it unpacked.

Good first issue: the docs and the status message.