Make line separator configurable

[djspiewak]

Yes, this is a Windows issue. :-(

It's relatively common on projects to standardize on Unix line endings even when doing development on Windows, mostly for the sanity of all involved. sbt-header mostly handles this in a reasonable way in every respect except for the fact that it appears to constantly try to recreate headers. The detection hard-codes System.lineSeparator() as the expected line terminator, and so what happens if you have a bunch of files with valid license headers but the "wrong" line ending is sbt-header will detect them all as needing new headers, then write a new header for each file using the overridden line endings (since we control the license contents), ultimately leaving the file unchanged. The practical effect of this is that sbt-header touches every file on every compile, leaving their contents unchanged but still re-written to disk.

This in turn requires workarounds like this to make sbt-header meaningfully usable across platforms.

If we had the ability to configure newLine, this really wouldn't be an issue. Or if there was at least a sanity check to ensure that newLine matched the line endings used in the configured license (usually \n).

[hseeberger]

PR welcome, Daniel ;-)