Support whitelist option for external helpers

[sorgloomer]

The babel-core function buildExternalHelpers supports a whitelist option (see https://github.com/babel/babel/blob/master/packages/babel-core/src/tools/build-external-helpers.js#L86), which is currently not used in this plugin (see https://github.com/rollup/rollup-plugin-babel/blob/master/src/index.js#L40).

Currently the asyncGenerator babel helper function is always included in the bundle, it does not get tree-shaken out, I guess because it's hard to detect the initialization does not have side effects. It could be worked around by enabling users to whitelist only specific helpers. It could even be made automatic later.

babel-core: v6.17.0
rollup-plugin-babel: v2.6.1

[sorgloomer]

This came up at StackOverflow. (http://stackoverflow.com/questions/40022087/using-the-whitelist-option-with-babels-external-helpers)

I created a pull request #92 with a quick workaround, however I am not sure about what the long-term solution should be.

[Rich-Harris]

Closing now that #92 is merged and released – thanks