Fstab syntax doesn't work as in 1.3 release

[gonzzor]

The expected fstab syntax doesn't work since commit 3da0b6c.

Something like this used to work with the 1.3 release.
/mnt/src /mnt/target fuse.rofs-filtered allow_other,ro 0 0

For some reason the source directory was moved from a positional argument to an option (-o source=) which make the fstab syntax fail.

See #36 for a pull request with my local changes restoring the old behaviour.

It was non-trivial to support the current design and the old 1.3 behaviour.