[Fix #1111] Fix reading of non-UTF-8 files in EndOfLine

The problem was that we used buffer.source.encoding as the file's
encoding when reading it again, but Parser has re-encoded the buffer
so it's always UTF-8. By first reading the file as ascii-8bit
(binary) encoding, and then re-encoding it based on Parser's logic
for recognizing encodings, we should be able to match /\r$/ always.
Merge pull request #1114 from jonas054/fix_1111_eol

[Fix #1111] Fix reading of non-UTF-8 files in EndOfLine
Add section about custom cops [ci skip]