Allow tidypy to run with a file, not just a folder

[laundmo]

This PR would allow tidypy to run on individual files too, not just a folder. Another step in my endeavour in getting it to work in vscode.

This is obviously a very basic and simple approach, and i fully expect having to change some things. Documentation also has not been updated at all, only the docstrings.

All tests passed with this, and trying to run a normal check on folder and file works.

[laundmo]

slight issue: this seems to prevent pyproject.toml from loading correctly. im investigating, dont merge.