Fix panic when printing the help, version, or specification output fails

[17krishna8]

The help, version, and specification outputs are printed with print!/println!, which panic when the write fails (for example with a broken pipe or a full disk) instead of returning a write error like the encode and decode modes do:

$ data-encoding --mode=specification --base=64 | head -0
thread 'main' panicked at library/std/src/io/stdio.rs:1168:9:
failed printing to stdout: Broken pipe (os error 32)

$ data-encoding --version > /dev/full
thread 'main' panicked at library/std/src/io/stdio.rs:1168:9:
failed printing to stdout: No space left on device (os error 28)

$ data-encoding --help > /dev/full
thread 'main' panicked at library/std/src/io/stdio.rs:1168:9:
failed printing to stdout: No space left on device (os error 28)

This replaces those macros with write!/writeln! on std::io::stdout() so that write errors are reported like the other modes:

$ data-encoding --mode=specification --base=64 | head -0
data-encoding: Write error: Broken pipe (os error 32)

$ data-encoding --version > /dev/full
data-encoding: Write error: No space left on device (os error 28)

Since all three outputs end with a newline, the line-buffered stdout flushes before wrapped_main returns, so no flush error is dropped. Follows the pattern of #172.

[ia0]

Thanks!

I've also made sure there's no silent errors when flushing.