Disable encoding detection

[brunnerh]

The temporary file is written as UTF-8 but read with auto-detected encoding, which can be faulty. In my case the misclassification leads to broken umlauts (e.g. ü => ü).

An option to disable the encoding detection and just use UTF-8 would be nice.

(As a workaround I override ExternalEditor.prototype.readTemporaryFile at runtime 😅.)