A new flag `--munge [rename|filter]` controls how a name munging will work.
Name munging only applies to fields named '.', '..', containing a NUL byte, or containing a forward slash '/'.
The `rename` option (the default) will change '.' to '_.' and '..' to '_..'; each NUL byte turns into '_NUL_' and each slash turns into '_SLASH_'.
This greatly simplified policy means that spaces and other special characters should work just fine in field names now.
Save original names that don't work as filenames (e.g., `.` and `..`).
We restore these names as appropriate---if a file is `rename`d with the _same_ name, we leave it alone. But renames to fresh names destroy original names.
Resolves#29.
There is no way to create a file with such a name: the metadata is purely copied. There's a TODO note in the code to allow users to inspect and edit this metadata, but it's not worth exposing until somebody asks for it. (I mean, really, please don't use `.` or `:/` as a property name.)
