Expose index.js generation

[arichiardi]

Sometimes you want to generate an index.js that is appropriate for you compilation level, but you don't quite want to use the compiler provided by serverless-cljs-plugin.

For instance one wicked dev (me :smile:) might want to use a custom script, or clj -m cljs.main for compilation but still be able to use the index.js provided by the plugin.

I know this sounds odd, and I am opening an issue to see if it is completely out of the world or not.