* Use colorable: argument as the only coloring control

* Centalize color controling logic at Color.colorable?

There are 2 requirements for coloring output:

1. It's supported on the platform
2. The user wants it: `IRB.conf[:USE_COLORIZE] == true`

Right now we check 1 and 2 separately whenever we colorize things.
But it's error-prone because while 1 is the default of `colorable`
parameter, 2 always need to manually checked. When 2 is overlooked, it
causes issues like https://github.com/ruby/irb/pull/362

And there's 0 case where we may want to colorize even when the user
disables it. So I think we should merge 2 into `Color.colorable?` so it
can be automatically picked up.

* Add tests for all inspect modes

* Simplify inspectors' coloring logic

* Replace use_colorize? with Color.colorable?

* Remove Context#use_colorize cause it's redundant

https://github.com/ruby/irb/commit/1c53023ac4