Add js and css urls arguments to Dash.__init__. #302

We should add keywords arguments to add javascript urls and stylesheets to the Dash.__init__ instead of relying on app.scripts.append_scripts and app.css.append_css.

Example:

import dash

app = dash.Dash(
    js_scripts_urls=['https://cdn.google.com/google-analytics.js'],
    css_stylesheet_urls=['https://cdn.bootstrap.com/bootstrap.css']
)

Basically just lists of urls that are added as external_url to the appropriate resources (app.css or app.scripts).