Add file name as a flag

[0WN463]

• renamed --output to --output-dir (note: backwards incompatible)
• added --file flag
  • also supports - to output data to stdout
  • this is for future support for piping the output to another program, but we need to change the QR code and other details to be printed to stderr instead when we want this
• when both are used, the output-dir will be prepended to the filename

Examples:

# saves file with the file path of ./output
./qrcp receive  --file output

# saves file with the file path of ./dir/$FILENAME
./qrcp receive  --output-dir dir 

# saves file with the file path of ./dir/output
./qrcp receive  --file output --output-dir dir 

# dumps file to stdout
./qrcp receive  --file -

[claudiodangelis]

Hello, many thanks for this contribution. I need some more time to review it!
C