document --validate

I'm trying to setup automatic PR validation for my org's elm repos using `elm-format` (as well as some other tools).
The problem is that there is no easy way to fail if a format is required.

Currently we have to make it so that the build server sets up a proper git clone of the repo, then run the following bash script:
```BASH
yarn elm-format --yes
if [[ `git status --porcelain` ]]; then
exit 1
fi;
```

(in case you don't know - `git status --porcelain` will print nothing if there are no changes on the repo, or a list of changed files if there are any.)

I'd love it if there was a `--fail-on-format` flag (or similar) that would cause the tool to `process.exit(1)` when it detects that a format must be done on the file.

Start with the `yarn elm-format --yes` invocation described in the issue and inspect how elm-format reports whether formatting changes are needed. The work is done when a documented validation flag causes a nonzero exit status when formatting is required, while the existing formatting behavior remains available.

Written by the indexing model from the issue text.