Avoid multiple babel-polyfill loaded

[AllenFang]

I think this patch will actually fix #16. Just explain again in the following

for the users who wrote webpack with es6 and there are two way to run
first is using babel-node, second is node --harmony and add require('babel/register'); at entry point. For these both condition, it will load babel/polyfill at first in runtime, so we need add a flag to avoid bootstrap-loader to load babel/polyfill again.

So I thinks this patch can fix ERROR in only one instance of babel-polyfill is allowed issue correctly.
btw, I think I can make the code more clean in the beginning of src/bootstrap.loader.js, if I have free time, I will do another PR

Thanks

[alex35mil]

Looks good, merging. Thanks, @AllenFang 👍

[AllenFang]

@alexfedoseev, Thanks a lot :)