Report error when file does not end with a newline character

[andreasabel]

Lifted from #44 (comment).

The following violation type does not produce an error:

fix-whitespace/README.md

Line 21 in eb7062c

* Ensure that the file ends in a newline character.

Reason: After breaking the file into lines, we cannot observe anymore whether the last line ended in a newline character. So maybe we have to implement our own line-breaking function which gives an error if the last line does not end with newline.

[ulysses4ever]

"our own line-breaking function" sounds a bit too much. Isn't just checking the last byte of the file enough? If the byte is \n (modulo cross-platform considerations), then the file doe not violate that requirement.