fixed #51 for env variables in payload json file

[nitintecg]

Summary

Describe the goal of this PR. Mention any related Issue numbers.
Fixes #51
Added Code for accessing github context variables in payload json file
Updated Valid TestCase

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[filmaj]

Hey this is great, thanks so much!

I think this repo needs its settings tweaked as I don't think the integration tests ran on this PR (probably because the repo origin for the PR is not the root repo, for security reasons). Perhaps @stevengill you can enable repo maintainers to allow for PRs to run tests if we approve? I'd like to make sure that the integration test for payload file successfully interpolated the github event data into the message post to ensure this is working properly.