Ignore merge commits in generate changelog step

It would be nice to ignore merge commits whether via branch or PR.

The changelog will generally look something like:

[xxxxxx] Merge branch 'release/1.0.0'
[xxxxxx] Merge pull request #1001 from feature/facebook-upgrade
[xxxxxx] Facebook SDK upgrade
[xxxxxx] Merge pull request #1000 from feature/fix-bug-1
[xxxxxx] Another commit for bug 1
[xxxxxx] Fix something for bug 1
[xxxxxx] Merge branch 'master' into dev

All of the merge commits just seem to add unnecessary fluff since the actual commits are probably more important here.

Maybe the feature request here would be to add an input to ignore certain commits via regex?

Original issue here: https://github.com/bitrise-steplib/steps-generate-changelog/issues/3

Hello @audiomacksean!

Thank you for the feature request, we will take look at it soon.