Made the README nicer, and some other small tweaks

* Updated the README (syntax highlighting, more examples, more descriptive); see here: https://github.com/epitron/coolline/blob/master/README.md
* Added `Coolline.readline` (for `Readline.readline` compatability)
* Added `Coolline#bind` (for binding keys on the Coolline instance)
* Fixed a bug (when passing `default_line` to `#readline`, `default_line` wasn't dup'ed, which could clobber the caller's string)