code: function compile(schema, _meta) {
    var schemaObj = this._addSchema(schema, undefined, _meta);
    return schemaObj.validate || this._compile(schemaObj);
  }
docs: Create validating function for passed schema.
  @this   Ajv
  @param  {Object} schema schema object
  @param  {Boolean} _meta true if schema is a meta-schema. Used internally to compile meta schemas of custom keywords.
  @return {Function} validating function