Add possibility to force color output

The current version of colorize does not print any output when STDOUT is a tty.
This pull requests makes that behavior configurable.

```ruby
STDOUT.isatty #=> false
"str".blue #=> not colored

String.force_color!
"str".blue # => colored
"str".red # => colored
```
```ruby
STDOUT.isatty #=> false
"str".blue #=> not colored

String.force_color!
"str".blue # => colored
String.force_color!(false)
"str".red # => NOT colored
```

```ruby
STDOUT.isatty #=> false
"str".blue #=> not colored

String.force_color! do
   "str".blue # => colored
end
"str".red # => NOT colored
```