Fix config struct processing, add (much needed) tests

[atc0005]

Added

• Tests (partial implementation/coverage)
  • validate the most common code-paths (much more TODO)
    • config handling
    • logging setup
  • run tests as part of GitHub Actions Workflow
    • recursively
    • after getting deps, but before linting steps
      in order to allow early failure
  • update Makefile to run tests recursively, verbosely
• Fix configuration precedence handling
• Add string slice equality check from StackOverflow

Changed

• Fix configuration precedence handling
  • Config file loses to everything except default config settings
• config.logBuffer moved to logging.LogBuffer
  • internal detail exposed for general use
    • may change this later
• Updated code (where apparent) to be more test friendly
  • e.g., use io.Reader instead of filename so that we can use
    an in-memory TOML config file for testing config parsing
    and precedence validation
• Split config package into smaller files in an effort to make
  related code easier to manage
• Fail if requested config file not found
  • previously an error was logged, but execution was allowed
    to continue.
• Move default values to Getter methods
  • use those methods instead of directly dereferencing config
    struct fields in the majority of the code
  • use Getter methods to guard against nil dereferences
• Partial work to de-couple reliance on Config{} (see Consider breaking dependency between config package and other subpackages #170)

Deprecated

Both of these functions from the config package do not appear
to be needed any longer, but are being kept for a cycle in case
I change my mind:

• Config.SetDefaultConfig()
• Config.GetStructTag()

Fixed

• Fix configuration precedence handling
  • Config settings are merged properly, so that even default
    settings are allowed to override lower precedence config
    sources
• NumFilesToKeep default (didn't match v0.5.1 change)
• Add missing default switch statements along with error
  return codes for Set functions with specific valid option values
• README updates
  • cover config file flag, environment variables
  • config precedence corrections
• Multiple linting errors (with more that needs to be evaluated)
• Add missing exit if IgnoreErrors is false
• Handle unintended nil assignment
• Update GoDoc doc file
  • reflect config file support, including updated Help text

TODO

• Update multiple tests to use "tables" instead of hard-coding
  specific checks (which ends up being very lengthy)
  • e.g., TestValidate()

References

• refs Add config file, pre-config initialization logging support #156, Consider breaking dependency between config package and other subpackages #170
• fixes Make explicit configuration choices override those set earlier in load order #161