feat/custom fallback for language detection

[lwollenbergfuzzy]

Short text now automatically get assigned English language, which might not be suitable.
This behavior prevents the user to detect this and perform their own solutions.

Either return none and let the user handle it or allow for a custom fallback, for instance a callable that is a language detection by the user.
Another possibility is to allow to turn off the language detection. This can be done for the auto partition with languages=[""], but not all partitioners allow this, like the md_partition.