Use by default os.StdErr for output

[cipriancraciun]

I know that with #11 support for specifying the output stream was added, and thus it is technically possible to use os.StdErr.

However given that such a library is most useful in CLI tools and filters (like sed, grep, etc.) that are part of a pipeline, having status printed on os.StdOut by default would take many (at least in the *NIX) world by surprise.

For example today I've tried integrating this library without thinking for a second it would clobber the standard output...

Therefore I would suggest making os.Stderr the default. (At least in a future version like v4 if backward compatibility is a major issue.)

[cipriancraciun]

Or, if os.StdErr isn't made the default, at least detect if the os.StdOut is actually a TTY, else don't output anything.

[schollz]

@cipriancraciun Thanks. I'll leave this open and let people post reactions so I can gauge how others view this. Personally I like going to stdout.

BTW, as mentioned you can set to os.StdErr in options:

bar := progressbar.NewOptions(10, progressbar.OptionSetWriter(os.StdErr))

[schollz]

As of v3.3 the stderr is default for the Default() and DefaultBytes() functions