add custom module formatters

MODULES.md

@@ -108,3 +108,31 @@ export function bar() {
   }
 });
 ```
+
+## Custom
+
+```javascript
+module.exports = ModuleFormatter;
+
+function ModuleFormatter() {
+
+}
+
+ModuleFormatter.prototype.import = function (node, nodes) {
+  // node is an ImportDeclaration
+};
+
+ModuleFormatter.prototype.importSpecifier = function (specifier, node, nodes) {
+  // specifier is an ImportSpecifier
+  // node is an ImportDeclaration
+};
+
+ModuleFormatter.prototype.export = function (node, nodes) {
+  // node is an ExportDeclaration
+};
+
+ModuleFormatter.prototype.exportSpecifier = function (specifier, node, nodes) {
+  // specifier is an ExportSpecifier
+  // node is an ExportDeclaration
+};
+```

lib/6to5/file.js

@@ -41,7 +41,16 @@ File.normaliseOptions = function (opts) {
 
 File.prototype.getModuleFormatter = function (type) {
   var ModuleLoader = transform.moduleFormatters[type];
-  if (!ModuleLoader) throw new ReferenceError("unknown module formatter type " + type);
+
+  if (!ModuleLoader) {
+    var loc = util.resolve(type)
+    if (loc) ModuleLoader = require(loc);
+  }
+
+  if (!ModuleLoader) {
+    throw new ReferenceError("unknown module formatter type " + type);
+  }
+
   return new ModuleLoader(this);
 };