ssub cases

I ran into a few cases that weren't handled by the code. One of them, you might well argue shouldn't be handled, but it allows code that uses the module to be a bit cleaner if the module takes care of this.

The first uncontroversial example just contains no dot, so:
/path/to/something_1-10#_exr

The second case is where the filename is the frame ID with no other characters (except the extension):
/path/to/0001.exr

The final case is a file that does not contain any # or @ placeholders:
/path/to/file.mov

You could argue that that's not a file sequence, or you could argue that it's a sequence of one file, but if the module were not to support that, I'd need to special-case it in the surrounding code.

Simple tests have been added to test/run.py for each of these.

Thanks!

D