Update rabbit to 2.1.9.

2.1.9: 2016-05-27

Markdown support improvement release.

Improvements

rabbit

* Suppressed warnings on Ruby 2.3. [GitHub#77][Patch by takiy33]
* Markdown markup: Supported strike through markup in GFM (GitHub Flavored
  Markdown). [Suggested by yoku0825]

  Syntax:

	~~deleted text~~

* Markdown markup: Supported reporting an error for using HTML because using
  HTML isn't supported.
* Supported stopping search by escape key.

* RD markup: Supported pango block verbatim. You can text with Pango markup
  in the block.

  Syntax:

	# pango

	<span foreground="red">Red Text</span>

* Supported filtering Markdown files in file choose dialog. [GitHub#83][Patch
  by tSU_Root]
* Markdown markup: Supported reporting an error for using horizontal rule
  because horizontal rule isn't supported. [GitHub#84][Reported by tSU_RooT]
* Markdown markup: Supported list in list. [GitHub#85][Reported by tSU_RooT]

Theme

* default-title-text: Supported customizing author font size by
  @title_slide_font_size.
* background-image-toolkit: Supported bottom vertical align.
* image-viewer: Supported comment.
* syntax-highlighting: Supported diff format.
* syntax-highlighting: Stopped highlighing numbers.
* clear-blue: Supported frame color in syntax highlight.

Document

* Added links to licenses. [GitHub#86][Patch by tSU_Root]
* Updated document about how to write a slide in Markdown. [GitHub#89][Patch
  by tSU_Root]

Fixes

rabbit

* Fixed a bug that slide with invalid format crashes Rabbit.
  [GitHub#76][Reported by takiy33]
* Fixed a bug that --margin option value is parsed wrongly.
  [GitHub#82][Patch by zunda]

Theme

* image: Fixed a bug that image size reduced with Markdown markup with
  image-timer theme. [GitHub#78][Reported by TOMITA Masahiro]
* image: Fixed a bug that image size isn't stable for image with caption.
  [GitHub#88][Reported by TOMITA Masahiro]

Thanks

* takiy33
* yoku0825
* TOMITA Masahiro
* zunda
* tSU_Root


(taca)