funk: Extend BOM warning to non-POSIX scripts

[mcandre]

As a script author, I want stank to warn when any file begins with a Unicode BOM before the shebang line, so that I can write more portable scripts.

If a file has a POSIX shebang line, but features a BOM before the shebang line, then have funk present a warning, regardless of whether the file is POSIXy.