Skip encoding, if the output file is present and --force-overwrite is not given

[luziferius]

Currently, retaining the temporary data is required for skipping already encoded input files. This affects the encoding process, if multiple input files are given.

If --keep-temp is not given, the temporary data will be wiped at the end of each encoding process. If then the process is aborted, either by killing the main process or by hitting the --limit-encodes limit, the already encoded files will be encoded again, which is a bug.

TODO: When processing an input file and --force-overwrite is not given, check if the output file is already present. If so, skip creating the temporary directory and all processing related to the file.