Add optional progress output for writing/reading flash

[ygoe]

This change adds the command line paramters -p and --progress that will output a progress indicator while writing and verifying the flash. It's a simple percentage value that is overwritten on the same line.

I hope this feature is interesting, I like it because the whole process can easily take a minute.

[MCUdude]

Is there any reason why this couldn't always be enabled? I mean, why would you not want to see a progress bar?

[ygoe]

Usually such self-updating output is optional. At least that's what I've seen so far everywhere. Anyway, there doesn't seem to be much interest in seeing the progress according to the age of this PR.