Use coloring only when writing to streams on a TTY

Dear Dimitris,

thanks a stack for conceiving and maintaining colored. We are using it successfully within grafana-wtf.

Now, Simon Malave told us through [1]:

Hi, I am using this tool and needed to export the output to a plain text file to upload it to a Jira issue. The text was bloated with special characters used to give colors to the terminal.

On this matter, I just discovered [2], which outlines a convention to set an environment variable NO_COLOR to suppress ANSI output. Apparently, this already gets honored by a bunch of software out there.

So, I wanted to humbly ask if you see that would also make sense for the colored package to support?

With kind regards, Andreas.

[1] https://github.com/panodata/grafana-wtf/issues/9
[2] https://no-color.org/