Convert raw command line arguments to --file-uri and --folder-uri

[owtaylor]

Calling Visual Studio Code with

'--remote attached-container+' <PATH1> <PATH2>

Resolves <PATH1> and <PATH2> to folder-or-file based on heuristics before passing them to the remote container. So it's better to check locally whether paths are folders or files and use the --file-uri and --folder-uri arguments.

Fixes: #14