Re enable custom formatters

[ericmcbride]

• A recent change caused Custom Formatters to not work with concurrency. I dont think that the godog library should not allow the usuage of concurrency on Custom Formatters.
• Custom Formatters not compatible with concurrency #237
• https://github.com/cucumber/godog/pull/204/files#diff-12f3273da1625fbe0b8ae7f4eefe2c3cR280

[codecov]

Codecov Report

Merging #238 into master will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           master    #238   +/-   ##
======================================
  Coverage    82.5%   82.5%           
======================================
  Files          16      16           
  Lines        2663    2663           
======================================
  Hits         2197    2197           
  Misses        332     332           
  Partials      134     134

Impacted Files	Coverage Δ
run.go	79.86% <0%> (ø)	⬆️

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[aslakhellesoy]

Hi @ericmcbride,

Thanks for your making your first contribution to Cucumber, and welcome to the Cucumber committers team! You can now push directly to this repo and all other repos under the cucumber organization! 🍾

In return for this generous offer we hope you will:

• ✅ Continue to use branches and pull requests. When someone on the core team approves a pull request (yours or someone else's), you're welcome to merge it yourself.
• 💚 Commit to setting a good example by following and upholding our code of conduct in your interactions with other collaborators and users.
• 💬 Join the community Slack channel to meet the rest of the team and make yourself at home.
• ℹ️ Don't feel obliged to help, just do what you can if you have the time and the energy.
• 🙋 Ask if you need anything. We're looking for feedback about how to make the project more welcoming, so please tell us!

On behalf of the Cucumber core team,
Aslak Hellesøy
Creator of Cucumber