Append only new files

When appending files, check if that file is already in the label file.
Do not append it if it's already there.
Don't append duplicate files on cmd line multiple times

If a file is specified multiple time on the command line, do not
append it multiple times.
Append only new files

When appending files, check if that file is already in the label file.
Do not append it if it's already there.
Merge branch 'feature/append-only-new-files'
Add a newline add the end of the JSON file

Editors like Vim add a newline at the end of the file when saving. git
then complains loudly about the "change":

  -]
  \ No newline at end of file
  +]

Add a newline to the JSON file to be compatible with editors.
Merge pull request #83 from neingeist/fix/json-newline-at-end

Add a newline at the end of the JSON file
Merge pull request #81 from neingeist/feature/append-only-new-files

Append only new files
Merge remote-tracking branch 'cvhci/master'