Accept pipenv's Pipfile.lock in place of pip's requirements.txt

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When running **shiv** to build an app file, a good way to include all the other packages your app depends on seems to be to pass in the option, `-r requirements.txt`. The `requirements.txt` file is managed by **pip**. But I use [`pipenv`](https://pipenv.pypa.io/en/latest/) in preference to **pip**. **Pipenv** lists dependencies in a file named [`Pipfile.lock`](https://pipenv.pypa.io/en/latest/basics/#example-pipfile-pipfile-lock).

I would like to have **shiv** accept `Pipfile.lock` in place of `requirements.txt` as a way to list all the packages which **shiv** should install into the app file.

Start by tracing shiv's command-line handling of the -r requirements.txt option and how that file is passed to dependency installation. Compare the expected Pipfile.lock format with requirements.txt, then verify that a Pipfile.lock can supply the packages used to build the app file.

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