Update webpack configuration documentation to suggest searching for loader

[peterp]

Right now we suggest people modify the webpack configuration via an index, but that means that we have to keep the indexes stable over time (not too much of a problem), but it would be nice to provide a utility function that allows people to search for a loader and modify it.

[thedavidprice]

@peterp are you thinking to provide the utility function or just give an example via documentation?

[peterp]

@thedavidprice I'm thinking of providing a utility function, and to update the docs.