Skip non-UTF-8 files

[kokkonisd]

The program crashes when it encounters a non-UTF-8 file.
It would probably make more sense to skip such files; when we run misspellings on a huge list of files it's frustrating to have it fail because maybe 0.01% of those files are non-UTF-8.
A possible solution could be to emit a warning when a non-UTF-8 file is encountered, with the option to silence such warnings.