Write out valid bmp files with timestamped names

[acmiyaguchi]

I ran the following command, and ran issues with the resulting file being an invalid image:

yarn ts-node bin/client.ts run \
  --uri wss://littleprinter.nordprojects.co/api/v1/connection \
  -p printer-zero.printer \
  --driver filesystem

This converts the buffer into a bitmap before writing out the file. I modified the name of the output to the current iso-formatted date to keep track of multiple prints.