Add %L format code (temporary copy of filelist)

The most salient use is cloning a feh session. See issue #128 for more information.

This turns out to have been very straightforward to implement.

As I indicated in the issue (issue #128), copies of the filelist are reused within a given command (so the two instances of %L within the action "echo %L %L" evaluate to the same string for a given invocation, but what %L evaluates to will change between invocations..). This is what I regard as the least surprising behaviour.