Dash

Fixed
- [473](https://github.com/plotly/dash/pull/473) Fix `_imports_.py` indentation generation.

Added
- [451](https://github.com/plotly/dash/pull/451) Combine `extract-meta` and Python component files generation in a cli

Fixed
- Fix a bug [renderer66](https://github.com/plotly/dash-renderer/issues/66) in the ON_PROP_CHANGE callback where history was not correctly set when acting on more than one component. In particular, the 'undo' button should now work as expected.

Added
- [362](https://github.com/plotly/dash/pull/362), [renderer#73](https://github.com/plotly/dash-renderer/pull/73) Hot reloading from the browser.
- Silence routes logging with `dev_tools_silence_routes_logging`.

Added
- [444](https://github.com/plotly/dash/pull/444) Add component namespaces registry, collect the resources needed by component library when they are imported instead of crawling the layout.

Fixed
- [450](https://github.com/plotly/dash/pull/450) Use the same prop name black list for component generation in all supported Python versions. Closes [#361](https://github.com/plotly/dash/issues/361).

Fixed
- [443](https://github.com/plotly/dash/pull/443) `Dash.registered_paths` changed to a `collections.defaultdict(set)`, was appending the same package paths on every index.