Feature request: accept PathLike object as I/O filename

[fepegar]

I have recently started using Python 3's built-in pathlib module (introduced in PEP 428 for path managing in my projects and I really like it.

I thought that it would be nice if nibabel accepted PathLike objects as filenames, instead of only str or whatever the current behaviour is.

In my code I use wrappers for nib.load and nib.save that always transform the filename argument to str. That might be naive, but it seems to work. As a reference, matplotlib uses os.fspath if the filename argument is PathLike.

[fepegar]

Closing, since I just saw that there is an open PR for this: #610

I'll reopen if it's not merged.