feat: allow user to disable log level validation

[emelski]

With this change, the user may specify validateLogLevels: false when instantiating a logger in order to disable validation of log levels. This is helpful in cases where the log levels are not known in advance. If validateLogLevels is true, the logger behaves as it does now: unrecognized values for the log level are detected and the associated line is rejected, or the log level is coerced to the default value.

• lib/logger.js: Add support for validateLogLevels option.

• test/common/create-options.js:

• test/logger-instantiation.js:

• test/logger-log.js: Add tests for validateLogLevels.

• README.md:

• types.d.ts: Update readme and type definitions.

Fixes: #65

[emelski]

Closing this as this approach is incorrect, and my need for the change has faded.