Scan for matching frames if no explicit framespec is provided.

Fixes #713.

This is a useful feature that scratches an itch. Basically the ability to do:

    $ oiiotool foo.#.exr -o bar.#.exr

in a directory containing:

    foo.0001.exr foo.0002.exr foo.0003.exr foo.0004.exr foo.0005.exr

results in the sequence:

    bar.0001.exr bar.0002.exr bar.0003.exr bar.0004.exr bar.0005.exr

Any time framespecs are explicitly provided they are used. If they are not used, then inputs are scanned for matching frames. 

If no framespec is provided for an output, the frame numbers from the first input are used.

This is backwards-compatible (at least, I think it is) with current oiiotool behavior except for one edge case: currently (I mean in oiio master), if no framespec is provided for any sequence, it defaults to "0".