Add the defaultSeverity property to the schema for tslint.json

[BehindTheMath]

The TSLint documentation has a property that is missing from the schema.

defaultSeverity?: "error" | "warning" | "off": The severity level used when a rule specifies a default warning level. If undefined, “error” is used. This value is not inherited and is only applied to rules in this file.

Suggested fix:
Add the following object to the properties section at the end of the schema:

"defaultSeverity": {
  "type": "string",
  "enum": ["error", "warning", "off"],
  "default": "error"
}