Readme no docker & allow using platform default version

[2bndy5]

• re-write section of README about using the action source code as a python package.
  • updated example and added example about CLI args compared to action input options.
• resolves tidy-checks always applied nevetherless local .clang-tidy #64 about using clang-tidy with only a config file.
• allow version input option to be blank.
  • When using the docker, the version default is 10.
  • When using the python CLI approach, the version is a blank string (allowing the platform's default installed version to be invoked).

[shenxianpeng]

LTGM. Especially I like the example about CLI args compared to action input options.

I just don’t know whether the code change will break something because we don't have a full regression test.

Maybe I can move the test-cpp-linter-action to this org and add some test cases for testing in short term.

Maybe we could add some unit tests for Python code in the long term.

[2bndy5]

I thought you'd eventually transfer the test repo over also. I'd like to merge the cpp-linter/test-cpp-linter-action#10. The updated workflows will be a good base for testing with and without the docker image.

I also agree that the code changes should be tested. The idea to change the version default (for only python CLI) came to me as I was re-writing the example for this PR. We needed a way to explicitly allow using the default version of clang tools in the platform's PATH env var.

[2bndy5]

I tested the version default in this CI run. It worked like a charm (and 15 seconds faster).

The changes to version option doesn't have an affect when using the docker image because the action.yml still passes 10 as the default value when the user doesn't specify a value (verified as such with this CI run). These changes to the version option only affect running the action as a python executable.

Other than those changes, the README update is the only thing left to review.