Add support for Webpack 3

[jnwng]

hello!

first of all, really appreciate the work here, once i found it i realized that i was the perfect solution for our performance issues building webpack applications in different locales. however, i had some trouble integrating it with webpack 3 (as well as supporting alternative resource paths).

while i have it working, im not sure what's the best interface for those improvements here (and if you even want to deal with webpack 3 support). you can see my fork here.

two things i added:

1. an option to replace the resourcePath. we have a system here where we call
   require('i18n!nls/page.js') which should farm out to require('i18n!nls/{en,es,ru}/page.js'), whereas it looks like this was designed to do require('i18n!nls/en/page') -> require('i18n!nls/ru/page')
2. the chunk output format was changed and so the string replace had to change as well.

if this support looks interesting to you, i'd be happy to split these two features into a PR here!