Auto-detect elm version with --stdin

(Continued from #561)

What to do in case of `--stdin` is a good question.

Editor plugins are easiest to implement using `--stdin` for multiple reasons. The main one is that people often format code in the editor before saving the file, in which case the file contents would not up be to date. In Emacs, I launch the `elm-format` subprocess so that its current directory (cwd) is the directory where the formatted `.elm` file resides, so looking up `elm.json`/`elm-package.json` based on the cwd would work fine. Not sure what is done by other editor plugins.

The best alternative might be to have a command line flag like this (copy-pasted from [here](https://github.com/Unibeautify/unibeautify-cli/issues/18#issue-335917742)):

> Something that's really useful for editor plugins is an option to say what filename the contents of stdin are coming from.
>
> * [prettier](https://github.com/prettier/prettier): this option is called `--stdin-filepath`
> * [rufo](https://github.com/ruby-formatter/rufo): this option is called `--filename`
>
> In my experience, it's easiest and most reliable to implement an editor plugin by not using disk files at all. Instead, just feed unformatted code to stdin and get the formatted code back from stdout. But even when using stdin/stdout, it's useful if we can know the filename of the code (the code usually comes from a disk file, but may have been modified in the text editor and the modifications have not been saved, so the disk file is different from the editor buffer that is sent to stdin).
>
> The point of knowing the filename is that:
>
> * Formatter error/warning messages can show a filename that is intuitive to the user.
> * Unibeautify can find `.unibeautifyrc` and possibly other project settings files based on the filename.

If `--stdin` is given, you could have an error message like this:

> I couldn't figure out what version of Elm you are using!
>
> With `--stdin`, you should use either `--elm-version` to specify the Elm version directly, or `--stdin-filepath` to let elm-format know what disk file or directory corresponds to the code being formatted. elm-format will then look for `elm-package.json` or `elm.json` in that directory (and its parent directories) to figure out whether you are using Elm 0.18 or Elm 0.19.

As a final twist in the tale, people should be able to format code in temp buffers that don't have a filename associated with them. In that case I don't think there is anything sensible that can be done apart from requiring a manual `--elm-version` flag along with `--stdin`. The concept of a current directory is dubious for a temp buffer.

Start with the existing --stdin handling and the discussion in #561; trace how elm-format parses command-line options and determines the Elm version. Compare the proposed --elm-version and --stdin-filepath behavior, then verify the error and help behavior for stdin input with and without an associated filepath.

Written by the indexing model from the issue text.