Respect output.filename config option

[kayakyakr]

Fixes #2

[benbender]

Just as a note: it's not that easy, because webpack accepts placeholders as the filename option.
F.e. this is valid in webpack:

options.filename = '[name].js';

In this case, your change will not be able to find the filename to start.

[kayakyakr]

Yeah, that's a good point, but the systems using the templated filename won't work in the current setup anyway, so you're not losing anything.

Will have to determine if it's possible to parse those templates to discover the file.

[kayakyakr]

Yeah, just reiterating the point that options.filename = '[name].js' won't work with the current setup anyway.