feat: add option to exclude merge commits

[arnested]

This adds an option to the init and add commands to exclude merge commits from the changelog.

Merge commits often contain no useful information for the changelog, and can be confusing to users. This option allows them to be excluded.

The default behavior is unchanged, and merge commits are included in the changelog.

The pull request does not include tests yet. I will add them if this is a feature you would like to see in the tool. I'll also add documentation if we move forward with the change.

The change is a breaking change for those using it as a library. I couldn't see another way to add the option without breaking the API. I'm happy to discuss other options if you have any ideas.

At least it doesn't appear to break any of the known, public uses of the library listed on https://pkg.go.dev/github.com/goreleaser/chglog@v0.5.0?tab=importedby.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (819eaf2) 65.18% compared to head (a3c8c51) 65.60%.

Files	Patch %	Lines
add.go	0.00%	0 Missing and 1 partial ⚠️
init.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   65.18%   65.60%   +0.41%     
==========================================
  Files           7        7              
  Lines         247      250       +3     
==========================================
+ Hits          161      164       +3     
  Misses         64       64              
  Partials       22       22              

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[arnested]

I have added some tests for the feature and mention the option in README.