Dash-devices

Added
- `Dash.get_asset_url` will give the prefixed url for the asset file.

Fixed
- [351](https://github.com/plotly/dash/pull/351) Prefix assets files with `requests_pathname_prefix`.

Fixed
- [343](https://github.com/plotly/dash/pull/343) Only create the assets blueprint once for apps that provide the same flask instance to multiple dash instances.

Fixed
- [336](https://github.com/plotly/dash/pull/336) Fix bug in `_validate_layout` which would not let a user set `app.layout` to be a function that returns a layout [(fixes 334)](https://github.com/plotly/dash/issues/334).

Added
- [318](https://github.com/plotly/dash/pull/318) Add `assets_ignore` init keyword, regex filter for the assets files.

Fixed
- [335](https://github.com/plotly/dash/pull/335) Ensure CSS/JS external resources are loaded before the assets.

Added
- [322](https://github.com/plotly/dash/pull/322) Take config values from init or environ variables (Prefixed with `DASH_`).

Fixed
- Take `requests_pathname_prefix` config when creating scripts tags.
- `requests/routes_pathname_prefix` must start and end with `/`.
- `requests_pathname_prefix` must end with `routes_pathname_prefix`. If you supplied both `requests` and `routes` pathname before this update, make sure `requests_pathname_prefix` ends with the same value as `routes_pathname_prefix`.
- `url_base_pathname` sets both `requests/routes` pathname, cannot supply it with either `requests` or `routes` pathname prefixes.