It would be great if the generator doesn't replace the entire file but just adds missing versions and ignores existing ones. We have some manual added information which we don't want to lose when generating the changelog.

Features:

• Ability to keep manual added information for older version tags
• Reduced GitHub API calls due to reduce amount of data necessary to retrieve
• Stop/continue a changelog generation process due to partial updates

Implementation:
The CHANGELOG.md needs to be parsed and divided into sections which represent the versions, e.g.

[{ :version => 'v0.0.1', :content => '...' }, { :version => 'v1.0.0', :content => '...' }, ...]

Missing versions can be fetched from the GitHub API and appended.

This might be related to #172.