Create default settings yml for config install

The exact set of starter rules has not been determined yet. This ticket is blocked until we have decided on the set of rules.

In addition to the rules defined in default settings above, I'd check for the following (examples of bad alt text I've encountered in the wild):

• file extension
• hyphenated text that doesn't match file name ("red-car-at-stoplight")
• keywords: "placeholder", "screenshot"

Thanks maggiewachs, that hyphenated one is interesting. I am not sure how to even support that one. We could probably use regex to look for words with too many hyphens but what is too many hyphens? Greater than one per word?