Fix pre-commit hook when called with multiple files

[abravalheri]

The hook added by #40 seems to be called with all the files that match the file name pyproject.toml. However the CLI tool is only able to handle 1 file at time.

This change tries to fix that by allowing the CLI to validate multiple files.

Additionally, since this is the most common use case, the files regular expression is changed to only match pyproject.toml in the project root.

@wwuck, please let me know if you have any objections here.

[abravalheri]

I am testing the new hook in one of my projects (pyscaffold/pyscaffold#611) that contains other pyproject.toml files (e.g. inside the tests directory).

[wwuck]

I haven’t tested but looks ok to me. Most projects will only have one file in the top project directory so it shouldn’t be a problem