Better mapping.

Works with

``js
$ = require('gulp-load-plugins');
// $.map(fn)
`

When returning a promise, it is mapped asynchronously (in parallel):

`js
var Q = require('kew');
// ...
.pipe($.map(function(file){
return Q.delay(500).then(function(){
file.tags = (file.tags || []).push('slow');
return file;
});
}))
`

The stream 'end' event delayed until all promisses are finished.

When you return undefined from your promise or regular function, the file is
filtered from the stream.

`js
.pipe($.map(function(file){
if(file.path.match(/gulpfile\.js/))
return file;
// other files not emitted
}))
`

That's it.

Alternatives

For regular callback style mapping, just use map-stream.

`js
var map = require('map-stream');
// ...
.pipe(map(function(file, done){
done(null, file);
}))
`

For both data and end events, through2 works well:

`js
var Through = require('through2').obj;
// ...
.pipe(Through(
function handle_data(file, encoding, done) {
this.push(file);
done();
}
function handle_end(done) {
done();
}
))
``