Add a new `annotator.config.settingsFrom#hostPageSetting(settingName)`
method that encapsulates a bunch of logic in one method:

- It always returns null if the client is from a browser extension.
  Browser extensions don't read settings from the host page.
- If the host page's `window.hypothesisConfig()` function returned a
  value for the requested setting, `hostPageSetting(settingName)` returns
  that value.
- If `hypothesisConfig()` doesn't return the setting it looks for it in
  `js-hypothesis-config` scripts in the host page
- If the setting isn't defined in either place it returns `undefined`.