Add `fly.imports` key to `package.json` config

Pulled from #233

For the sake of reusable, private plugins:

{
  "fly": {
    "imports": [
      "./build/custom-task-one.js",
      "./build/custom-task-two.js",
      "./build/custom-task-three.js"
    ]
  }
}

// build/custom-task-one.js
module.exports = function () {
  this.plugin('plugOne', {every: 0}, function * (files, opts) {
    // custom stuff
  })
}

// flyfile.js
yield this.source('src').plugOne({hello: 'world'}).target('dist');

---

Open to alternative names for the imports key.

The same functionality can be achieved using npm link (or npm install and file:./ as a dependency), but this requires that each plugin has its own package.json file.

You can group multiple plugins in the same “package” (example). However, this method also doesn’t scale well & becomes quite messy to manage.

[lukeed]

@devmondo You can now access this by installing the latest beta release.

[devmondo]

man lovely!!! i cant wait for this to be released