Support a function that retruns a stream of tasks instead of an array of tasks

[futurechan]

Instead of

tasks:[less(), minifyCss(), 'concat'],

we should support something like

tasks:function(files){
  return files
    .pipe(less())
    .pipe(minifyCss())
    .pipe(concat());
}

[futurechan]

I want to do a few more things before I publish 1.5:

• [x] introduce tasksStrategy and streamStrategy modules to encapsulate logic
• [x] add documentation