Support user-provided end-of-line characters

[baronfel]

This PR implements:

• A new configuration setting for the user-supplied end_of_line setting
• Parsing for this setting
• Tests around the parsing
• Use of this setting to generate newlines

There are currently some bugs in the replacement of newlines, specifically around adding newlines on single-line code samples, and we have questions about the use of the removeTrailingSpaces function, specifically if its functionality could be rolled into a more direct manipulation of the WriterModel.

[nojaf]

@baronfel I finished the implementation.
One thing though:
--check flag should not consider the line endings as a change (see #1196)
But formatting with a file with \r\n using setting lf should write the changed file to disk, so that is why I had some changed to the FAKEHelpers.