Add uncolor String method to remove ANSI color codes

[duffyjp]

I added uncolor to String. It's the same name and code used in Rainbow.

I often find it handy to strip color codes from terminal output to use in emails etc where it's not interpreted.

Example:

2.6.5 :003 > "foo".red + "bar".blue
 => "\e[1;31mfoo\e[0m\e[1;34mbar\e[0m" 
2.6.5 :004 > ("foo".red + "bar".blue).uncolor
 => "foobar" 

[HarlemSquirrel]

Hey there, thanks for the contribution! The failing spec should be fixed in master so could you please update your branch when you have a moment?

[HarlemSquirrel]

@duffyjp Do you have a moment to update this branch from master?

[HarlemSquirrel]

Thank you!