adds selectors to the Audit results

[philschatz]

This adds an additional field to the Audit results object which provides an array of selectors that were found. I added it as an additional field to maintain backwards-compatibility with previous versions.

Some use-cases for adding selectors:

• running audits as a part of unit/integration tests could use the selector to provide a more helpful message (see electron/spectron#126)
• tools like electron/devtron#87 could use the selector directly to inspect(...) an element when clicked

Questions

1. Since getQuerySelectorText(element) does not always generate a valid selector, should it be configured in the AuditConfig via something like config.includeSelectors = true?
2. Oh dear, it looks like Google forgot that I signed the CLA; how should I proceed?

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