tools: improve docopen target in Makefile

1. As it is, it just tries to build only the `all.html` file. If none of
   the other files are built already, generated page will not be good.
   To fix this, we process the assets and generate HTML files first.

2. After the HTML is generated, `google-chrome` is used to open the
   generated file in browser. This is not very portable as it might not
   be installed or installations might have used a different name. So,
   we use Python's webbrowser module to open the file.

PR-URL: https://github.com/nodejs/node/pull/9436



Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>