Add main file to improve usage with RequireJS optimizer

This change allows me to do this:

``` js
pipelines['sbscribe:dev:js'] = [
  sbscribeBower('argonaut'),
  requirejs(),
  write('sbscribe-front-end/static/js/other-libs')
];
```

Without this change, passing a file name as the second argument to the Bower
operation would not suffice. In that case, it would pipe through the `src/api.js`
file (previously `main`) to RequireJS, which would then generate a bundle with
a module ID of `api`. I need the module ID to be `argonaut`.

Maybe there’s a better way of working around this. I did think about doing a
rename operation after the Bower op and before the RequireJS op, however I
believe that would fail as the rename op does not save the renamed file to disk
(could it?).