Add convenience API ColorScheme::stackTraceText(Throwable)

[remkop]

Currently, client code using ColorScheme::stackTraceText looks like this:

StringWriter sw = new StringWriter();
ex.printStackTrace(new PrintWriter(sw, true));
err.println(cmd.getColorScheme().stackTraceText(sw.toString()));

We can add a convenience API that accepts a Throwable, so that the above can be reduced to one line:

err.println(cmd.getColorScheme().stackTraceText(ex));