Bugfix: Must dup the `default_line`, otherwise the caller's string gets modified.
* Updated the README (syntax highlighting, more examples, more descriptive).
* Added `Coolline.readline` (for `Readline.readline` compatability)
* Added `Coolline#bind` (to bind keys on the Coolline instance only)
Used regular markdown syntax for code blocks for YARD to format it properly
README: One might as well perform in-place reverse directly