Support file extensions with multiple dot-components

[justinfx]

Refs justinfx/gofileseq#10

File sequence parsing does not support file extension formats that have multiple dot-components. Examples are:

seq_archive.1-10#.tar.gz
SimCache.100-200#.bgeo.sc

Current implementation would see the extension as ".gz" or ".sc" and drop the prefix components.
For sequences that have frames, we can assume that everything after the frame number or padding is the extension. We expect to parse ".tar.gz" and ".bgeo.sc".

For path patterns that do not contain a frame or padding, we can only assume the extension is the last dot-components: "file.with.multiple.info.ext" -> ".ext"