feat(options): add mergeCommitFilter option to cli

[mediavrog]

Based off the discussion on #375 and other issues, I started an implementation to add a new option to either include, exclude or only show merge commits when generating the changelog.

It still needs tests and I need to test the code myself properly, but so far I could already verify that the changelog is complete when just dropping the merges:false from the gitRawCommitOpts in merge-config.js.

For now, I think the option merge-commit-filter should default to exclude, as this is the current behavior and would not break existing usage.

I will try to clean things up later when I have more time, but please also feel free to contribute.

[coveralls]

Coverage decreased (-0.05%) to 92.665% when pulling 6c3b0ce on mediavrog:include-merge-commits into ab58e7a on conventional-changelog:master.

[coveralls]

Coverage decreased (-0.1%) to 92.585% when pulling e733449 on mediavrog:include-merge-commits into 49dcd6e on conventional-changelog:master.

[tommywo]

Hi @mediavrog and thank you for contributing. 🎉
I've added some notes on the code.

[mediavrog]

verified working using the below command on a project using git-flow (and thus tags on merge commits)

npm i -g ./packages/standard-changelog
standard-changelog --first-release -m include
standard-changelog --first-release -m only-merges
standard-changelog --first-release -m exclude
standard-changelog --first-release -m abc // default to exclude

[mediavrog]

@tommywo I need some help with the tests. If you could have a look at https://github.com/conventional-changelog/conventional-changelog/pull/595/files#diff-177d48f386b5a5dce18e8dd697f40b1dR304 and point me to what I am doing wrong.

Also, I'd ideally want to make the tests for ----merge-commit-filter independent from --first-release

[bcoe]

@mediavrog I apologize for the slow momentum on this project recently. Could I bother you to resolve the conflicts on this branch?

[RangerRick]

I tried doing a quick merge to see if I could get this working but the tests fail for me and I don't understand the internals enough to fix it. If someone has more time than I to look into it, my attempts at cleanup are here