Support for Nuke-style %V and %v wildcards in oiiotool

aka stereo support.

Examples:
`oiiotool foo_%V.%04d.exr` will match `foo_left.%04d.exr` or `foo_right.%04d.exr`.
`oiiotool foo_%v.%04d.exr` will match `foo_l.%04d.exr` or `foo_r.%04d.exr`.

The default views matched by %V are left and right, and the first character of each is used for %v. These may be overridden with the new `--views` option, for example `oiiotool --views LEFT,RIGHT ...`. The syntax is compatible with both Nuke and RV.

For this first pass, I only added to the public interface of the Filesystem namespace, so the non-view-supporting enumerate_file_sequence function is still present but unused. I dunno if this is used elsewhere, so I didn't want to remove it completely. But it should probably be deprecated, or just dropped.

The original scan_for_matching_filenames function is used by the newer view-supporting version, so it remains unchanged. It could go static, though, and keep the Filesystem namespace clean.

I added a few unit tests (all good) and also updated the docs.

This patch has been in production for a couple weeks now with no problems.

cheers,
-Mark