Add ability to write out the interactive HTML table with javascript to disk

To go along with ``t.show_in_browser``, it would be cool to be able to do:

```
t = Table()
...
t.write('table.html', format='jsviewer', include_js_files=True)
```

which would write out the HTML pages with the javascript files by the side, all ready to upload to a web server.

cc @keflavich

Eventually we'll be trying to reduce imports where possible related to I/O registry.  Is this import really needed here, or will we later be able to take it out?  In other words, the core table shouldn't need jsviewer straight away.