filesrc: Add a "loop" option to enable infinite streaming of a file

For a project I'm working on I needed to simulate an infinite video stream from a file. At first I used multifilesrc with a single file as it has a loop option, but soon found out that multifilesrc reads the entire contents of the file into memory, not desirable when streaming very large files.

The internet had a few suggestion of how to use filesrc, but it felt cleanest to add the loop option to filesrc.