Merge user's input config with pyta's default options

[sushimon]

Motivation and Context

The current configuration behaviour will completely overwrite all of pyta's default options if the user provides their own config options. As a result, the user must provide a lengthy config file containing all of pyta's default options, along with any overridden ones. These changes allows the user to provide a very small config file containing only the options they want overridden.

Your Changes

Description:

• Modified pyta's configuration behaviour to always load the default .pylintrc file first, before applying any user-specified configuration options, if provided.
• Added test cases to verify that both methods of applying configurations correctly update the linter's config options.
• Updated the documentation of configuration to provide more information about the new configuration behaviour changes.

Type of change (select all that apply):

• Refactoring (internal change to codebase, without changing functionality)

Testing

• Ran the test suite and verified the test cases passed.
• Ran the newly added test cases and verified they passed.
• Ran python_ta.check_all calls with an inputted config dict and a new .pylintrc file and verified the specified configuration options were correctly applied to the linter.

Questions and Comments (if applicable)

Checklist

• I have performed a self-review of my own code.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.
• I have added tests for my changes.
• I have updated the CHANGELOG.md file.

[coveralls]

Pull Request Test Coverage Report for Build 5315534521

• 18 of 18 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 89.498%

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Totals
Change from base Build 5236184120:	0.07%
Covered Lines:	1747
Relevant Lines:	1952

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💛 - Coveralls

[david-yz-liu]

@sushimon good work! I left one comment about the logic. But also I realized that to preserve backwards compatibility it would be good to provide an option to not use the PythonTA default config. That could be an optional argument to _check (and, transitively, check_all/check_errors) like load_default_config (with default value True).

[david-yz-liu]

@sushimon good job! I left another review but I think you're pretty close.

In addition to the inline comments, please make sure to add some tests for load_default_config=False to the test cases you added.