Allowing for dynamic destinations by accepting functions as the path …

[ktstowell]

One of my coworkers was trying to dump concatenated files into directories dynamically based on their origins. This PR attempts to cover that use case while maintaining existing functionality. This is what I added to the README:

---

Concat can also take a function for its first argument or as the value of path if using the object syntax.

function name(file) {
  return 'dest.js';
}

gulp.task('concat', function() {
  return gulp.src(['./scripts/**/*.js])
    .pipe(concat(name))
    .pipe(gulp.dest('./dist/'));
});

The function will be passed the current file from the stream. The most common use case for this is to output concatenated files into groupings based on their origination. The bottom line is that this allows you to add a destination dynamically.

To see an example, fork this repository and run the tests in ./spec.

[ktstowell]

Closing because I suddenly feel weird about my tests being outside of the existing runner. Will open once I integrate.

[yocontra]

This PR is doing a ton of stuff, can you outline what else is being added and split it up into multiple PRs?

[ktstowell]

Sure - after integrating the tests I realized it was a sizable change with how concat has fundamentally worked so I created #100 to see if this would even be a change you all would be interested in.