Add inputs

[2bndy5]

resolves #3
This adds 2 new inputs to the action:

1. style can be set to any of the values that clang-format accepts.
2. extensions a comma seperated list of file extensions for which to focus on. I added this one because I noticed the action was running clang-tidy/format on all changed files in the PR (which obviously resulted in more annoying failures and longer bot comments).

I can explain how the input args get passed to the run_checks.sh if needed.

I have also updated the Readme about the user inputs and required GH secrets token

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demo now lives here

I added a basic workflow using the action to test on a drafted PR since push events always fail with this action.

[shenxianpeng]

Hi Brendan, your PR looks amazing! I tried to run act pull_request locally, it seems success, I guess you should have tested it, so I am happy to merge.

I can explain how the input args get passed to the run_checks.sh if needed.

I would love to learn from you how to pass parameters to run_checks.sh, thank you.

[2bndy5]

The secret sauce is here in actions.yml:

https://github.com/shenxianpeng/cpp-linter-action/blob/180d0c2ee05e2fb21f3f237078471c130344c46f/action.yml#L19-L21

According to GH docs the args list replaces the Dockerfile's CMD option. Meaning, all args are basically strings passed to the Dockerfile's ENTRYPOINT executable as CLI args. From there I just used bash-script to capture them and then do something with them.

https://github.com/shenxianpeng/cpp-linter-action/blob/180d0c2ee05e2fb21f3f237078471c130344c46f/runchecks.sh#L8-L9

You can probably better understand now why I had to make modifications to the Dockerfile.