Adds custom reporter capability

[scalvert]

As part of #207, this PR adds a new reporter system that will allow us to provide custom reporting for accessibility violations for test runs.

The reporter conforms to the following interface:

interface A11yAuditReporter {
  (axeResults: AxeResults): Promise<void>;
}

A setCustomReporter is provided, allowing you to provide a custom A11yAuditReporter implementation.

export function setCustomReporter(
  customReporter: A11yAuditReporter = DEFAULT_REPORTER
): void;

This API provides the flexibility to shape the violations in any format you want.

[scalvert]

@rwjblue I'm gonna stop accepting your suggestions since they keep killing my formatting!! JK! Fixed the prettier errors.

[rwjblue]

Haha! FINE! 😝