Add configuration setting to control newline characters in metadata.

[featherbutt]

This adds the configuration option metadata.newline, which behaves like output.stdout.newline, but for generated metadata files.

The option is forwarded to calls to open when writing metadata.

Alternative solutions considered:

• Manually replace newlines in the metadata instead of having open do it, then passing newline='' to open
• Add a metadata.reconfigure field instead that gets passed to a call to TextWrapperIO.reconfigure

The current implementation is the cleanest and simplest, but has one downside: because the tests use mocks for calls to open, the tests can verify that the newline argument was passed but can't test the effect it has on the output.