Allow to specify entrypoint

[peacememories]

Hi, and thanks a lot for writing this tool and updating it for Elm 0.19
I'm planning on using it in my resource packing workflow. The plan is this:

1. Scan all the resources and code-generate Elm files that expose the resources as functions.
2. Scan the resource functions for usage
3. Only pack the resources that are actually used
   I think elm-xref can help a lot with this, but one thing I'm not clear on is how to find out which entrypoints use a certain function. This is important to me because our Elm codebase has many entrypoints, and they could become unused as the codebase changes, but may still contribute to the usage of a function.