Fix Issue #790

[bcheidemann]

This fixes issue #790 by exposing the FilerWebpackPlugin class through it's own import.

var { FilerWebpackPlugin } = require('filer/webpack');

It also deprecates the following:

var { FilerWebpackPlugin } = require('filer');

And updates the documentation in README.md to reflect these changes.

[bcheidemann]

@humphd what do you think?

[humphd]

This makes sense to me. @thomas-jakemeyn, do you have any input for your use case?