Improves the way the validator displays matches with many possible values

[rpaul-stripe]

When a matches rule has a large number of possible matches (common when validating to make sure that links point to a valid route or that images point to a file that exists) the error message can be extremely verbose. This PR makes it so that when the number of possible values exceeds eight, the validator only lists the first eight explicitly and then indicates how many more potential values are available. This ensures that users don't get giant error messages with hundreds of possible values.

The new error message looks like this:

Attribute 'jawn' must match one of ["a","b","c","d","e","f","g","h", ... 4 more]. Got 'cat' instead.

This approach loosely imitates the way that the node.js repl displays long arrays that are printed with console.log.