Update 'news' command to directly update the changelog

Author: brettcannon

.github/release_plan.md

@@ -8,7 +8,7 @@
 - [ ] Update [`CHANGELOG.md`](https://github.com/Microsoft/vscode-python/blob/master/CHANGELOG.md)
    - [ ] Create a new section for this release
    - [ ] Copy over the "Thanks" section from the previous release and make sure it's up-to-date
-   - [ ] Run [`news`](https://github.com/Microsoft/vscode-python/tree/master/news) (typically `python news --final | code-insiders -`)
+   - [ ] Run [`news`](https://github.com/Microsoft/vscode-python/tree/master/news) (typically `python news --final --update CHANGELOG.md | code-insiders -`)
    - [ ] Touch up news entries
    - [ ] Add any relevant news entries for ptvsd and the language server if they were updated
 - [ ] Update [`ThirdPartyNotices-Distribution.txt`](https://github.com/Microsoft/vscode-python/blob/master/ThirdPartyNotices-Distribution.txt) by running [`tpn`](https://github.com/Microsoft/vscode-python/tree/master/tpn) (typically `python tpn --npm package-lock.json --npm-overrides package.datascience-ui.dependencies.json --config tpn/distribution.toml ThirdPartyNotices-Distribution.txt`)