Add line-ending detection to linter

Seems like a bad meta.yaml with CRLF gummed up the works for a bunch of folks on staged-recipes… it seems reasonable to detect and warn about line endings.

[beckermr]

Yes. This is just for when the linter checks out the code so that it can find the offending line endings without git getting in the way.

[BastianZim]

Found another thing: https://docs.python.org/3/library/functions.html#open-newline-parameter

When reading input from the stream, if newline is None, universal newlines mode is enabled. Lines in the input can end in ‘\n’, ‘\r’, or ‘\r\n’, and these are translated into ‘\n’ before being returned to the caller. If it is ‘’, universal newlines mode is enabled, but line endings are returned to the caller untranslated. If it has any of the other legal values, input lines are only terminated by the given string, and the line ending is returned to the caller untranslated.

So setting the option '' should give us the unchanged output, if I understand this correctly.