feat(#166) Add path-group style router creation

[sombriks]

This PR is a proposition to fix #166

Please share comments and let me know your tougths, what features are missing, etc.

Checklist

• [X] I have ensured my pull request is not behind the main or master branch of the original repository.
• [X] I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
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[sombriks]

after some consideration it makes more sense as a standalone library https://github.com/sombriks/koa-api-builder