Specify file encodings

[klmr]

R source files don’t allow specifying an encoding. Since everything but Unicode is nonsense in this day and age, import should by default load all files as UTF-8 (to ensure that UTF-8 containing files work on Windows), see also #19.

The question is whether we can afford to be opinionated or whether import needs an argument to override file encodings, or whether we copy PEP-0263 and allow specifying the source file’s encoding inside the file.