function compile(schema, _meta) {
    var schemaObj = _addSchema(schema, undefined, _meta);
    return schemaObj.validate || _compile(schemaObj);
  }
Create validating function for passed schema.
@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