feature: support for json5 style comments

[glowinthedark]

jless throws an error with JSON5-style comments:

[
  {
    "operation": "shift",
    "spec": {
      "rating": {
        "primary": {
          // simple match.  Put the value '4' in the output under the "Rating" field
          "value": "Rating",
          "max": "RatingRange"
        },
        // match any children of "rating"
        // Shiftr has a precendence order when matching, so the "*" will match "last".
        // In this case anything that isn't "primary".
        "*": {
          // &1 means, go up one level and grab that value and substitute it in
          //  in this example &1 = "quality"
          "max":   "SecondaryRatings.&1.Range",
          "value": "SecondaryRatings.&1.Value",
          //
          // We want "quality" to be a value field in the output under
          //  "SecondaryRatings.quality.Id", but "quality" is an input key not an input value.
          // The "$" operator means use the input key, instead of the input value as ouput
          "$": "SecondaryRatings.&1.Id"
        }
      }
    }
  }
]

Comments are, of course, not a part of the standard JSON spec, but it would be great to be able to have some support for certain JSON5 extensions such as comments.