.flowconfig default filename if none provided in name_mapper

We're using webpack aliases, and modified our .flowconfig to match.

The name_mapper doesn't seem to have the ability to default to a specific file, like index.js if no filename is provided. E.G.

`module.name_mapper='^Modules\/\(.*\)$' -> '/src/modules/\1'`

If I have an index.js in /src/modules, ideally I would just be able to have my import like so:

`import { MenuBlock, OrderSummaryView } from 'Modules';`

This results in a flow error. I'd like to have the name_mapper infer that it should look for a default file I specify in name_mapper when none is provided so I don't have to use `'Modules/index'` in my import, but also retain the ability to specify specific files.