function setParser(type, parserFn) {
	if (typeof parserFn !== "function") {
		throw new Error("invalid parser function for " + type);
	}

	if (typeof parsers.get(type) !== "undefined") {
		warning("overriding parser for " + type + " format");
	}

	parsers.set(type, parserFn);
}
specify a parser/preload function for the given asset type
@memberof loader
@param {string} type - asset type
@param {function} parserFn - parser function
@see {@link Asset.type}
@example
// specify a custom function for "abc" format
function customAbcParser(data, onload, onerror) {
   // preload and do something with the data
   let parsedData = doSomething(data);
   // when done, call the onload callback with the parsed data
   onload(parsedData);
   // in case of error, call the onerror callback
   onerror();
   // return the amount of asset parsed
   return 1
}
// set the parser for the custom format
loader.setParser("abc", customAbcParser);