Dash

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.

Fixed
- [320](https://github.com/plotly/dash/pull/320) Disallow duplicate component ids in the initial layout.

Fixed
- Fix bug [321](https://github.com/plotly/dash/issues/321) where importing Dash components with no props would result in an error.
- Fix a bug in 0.23.1 where importing components with arguments that are Python keywords could cause an error. In particular, this fixes `dash-html-components` with Python 3.7.

Added
- [319](https://github.com/plotly/dash/pull/309) Add a modified time query string to assets included in the index in order to bust the cache.

Added
- [316](https://github.com/plotly/dash/pull/316) Add `ie-compat` meta tag to the index by default.
- [305](https://github.com/plotly/dash/pull/305) Add `external_script` and `external_css` keywords to dash `__init__`.
- Dash components are now generated at build-time and then imported rather than generated when a module is imported. This should reduce the time it takes to import Dash component libraries, and makes Dash compatible with IDEs.

Fixed
- [273](https://github.com/plotly/dash/pull/273) Raise a more informative error if a non-JSON-serializable value is returned from a callback.