Deactivation switch for ANSI color

[trans]

There needs to be a way to globally deactivate ANSI coloring.

[trans]

Should this effect all codes including moving the cursor, or just effect colorization codes?

[trans]

This has been implemented with $ansi global variable. It will work in almost all cases. But there are still a few edge cases that will need address in the future, and also it doesn't effect BBCode yet b/c that lib still needs to be fully integrated with ANSI::Code.

[trans]

It has been pointed out to me that it would probably be better if a global wasn't set by default. For that to work there are two options.

1. Either $ansi = nil would have to be equivalent to $ansi = true and only setting it specifically to false would deactivate it. That'll work but it is a bit odd to have nil === true.
2. Or we need to use a $noansi (or $no_ansi) global instead.

Both cases may cause backward compatibility issues --the later more so than the former. So does the benefit out way the downside of change?