Add autofix for W292 [NoNewLineAtEndOfFile]

[Sawbez]

I have added an autofix for W292, updated the tests, and updated the README. I'm relatively new to Rust and would love any feedback (even though there isn't many changes). If you would prefer, I could move the check into plugins.rs instead of keeping it in the same file. Also, if you want, I can open an issue as specified in CONTRIBUTING.md if this is considered bigger changes.

Unrelated note: I ran git push on my own machine with my own PAT (personal access token) and somehow on the first commit someone named "@kasryan" committed, but on later commits with the same command and PAT I was correctly shown as the person who committed. I have no clue who @kasryan is.

[charliermarsh]

Thanks for this -- looks great! No idea what happened with the multiple authors, that's extremely strange, but kind of funny :)