Do not overwrite human-readable changes to old versions

> This might be best left to external scripts, but I'm interested to see how people feel about it.

My ideal workflow would be:

1. To run `github-changelog-generator` on every merge into `master` as an automatic commit.
2. Directly commit human-readable changes to `CHANGELOG.md` in `master`.
3. Repeat.

Problem right now is that the human-readable changelogs would be overwritten by step 1. The idea of this feature request is to only add changes to the `CHANGELOG.md` file in versions that are older than the latest found tag.

An external script can do this:

1. Parse the `CHANGELOG.md` file using regex (or by creating a Markdown AST) and collect a file containing all of the sections of versions that have tags.
2. Create a `HISTORY.md` file using step 1's output.
3. Run `github-changelog-generator`. It will pull `HISTORY.md` by concatenating to the end of the file.
4. Delete `HISTORY.md` (optional).

Any thoughts on this? Thanks!