Support overriding `ensureCorrectName` (e.g. support `LICENSE.md` instead of `LICENSE`)

[titanism]

Can we please have a feature that lets us turn off this error if the file extension is present, e.g. .md?

https://github.com/hashicorp/copywrite/blob/807a790c13dfe5f09df5e7a3438dc09eb08487c0/licensecheck/licensecheck.go#L21