Ability to require packages in transforms

[timofei-iatsenko]

Hi, i use your tool for creating jscodemod transforms. It really useful, thanks for that.

But I faced situation when i need some package from npm in my transform, for example package for parsing jsdoc, and now i can't do it in the explorer.

So it will be nice to have some opportunity to connect external resources like in jsfiddle.

[fkling]

Duplicate of #165. I agree, it would be great 😃