zstdseek CLI improvements

[vasi]

The main thing stopping me from using zstdseek is it's CLI is a bit odd. Here are some things I'd like to fix/change:

• Error messages shouldn't be JSON, structured logging is great but not really human-readable. If stderr is a TTY, we should probably use zap's console encoding or some variant.
• Without -f or -o, zstdseek could just assume stdin/stdout like most compressors do.
• Configuring block size with -c is a bit complex. It'd be nice if -c N would be treated as an alias for something like -c N/4:N:N*4.
  • Even better, we could vary default block-size depending on compression quality. There's not much point asking for -q 19 with the default size.
• It'd be nice to list reasonable values for -q in the help, since many of them are just equivalent to each other.
  • It'd aso be nice to take -5 instead of -q 5 like most compressors do, but this may not be easy with go's flag module.
• Now that we have parallelism, we probably want a flag for number of CPUs.

@SaveTheRbtz Do these sound reasonable to you? I'm off on vacation shortly, but when I'm back I'd be happy to implement them.