Add mruby-range-ext

Added the following methods.

- Range#cover?
- Range#first (redefinition, accepts an optional argument)
- Range#last (redefinition, accepts an optional argument)

```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> (1..5).cover?(3)
 => true
> ("a".."l").cover?("m")
 => false
> (1..200).first(5)
 => [1, 2, 3, 4, 5]
> (1..200).last(6)
 => [195, 196, 197, 198, 199, 200]
> (1...300).last(4)
 => [296, 297, 298, 299]
>
```