Don't generate colorized output escapes when stderr isn't a TTY.

When piping to a pile, more/less, etc, this is generally undesirable.

This behavior can be overridden with the --colorized-output command-line
flag.

ispc.cpp

@@ -578,6 +578,7 @@ Globals::Globals() {
     disableWarnings = false;
     warningsAsErrors = false;
     quiet = false;
+    forceColoredOutput = false;
     disableLineWrap = false;
     emitPerfWarnings = true;
     emitInstrumentation = false;