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

@mdboom - is there a procedure for changing the default values for configuration items in the new system? Or is just this ok? Users that haven't customized the value should get the new default value, right?