Packing fails if output directory does not exist

[RaniSputnik]

If the output directory does not exist then the packing fails with open build/atlas-1.lua: no such file or directory. Because I want to be able to output to any kind of destination (see https://godoc.org/github.com/RaniSputnik/lovepac/packer#Outputter interface), not just the file system, solving this problem may be quite tricky.

I don't really want to add any more methods to the Outputter interface, perhaps it should be left as the callers responsibility.