Implement preview feature for creating and editing issue

Javascriptライブラリmarkedを使ってプレビュー機能を実装。
chjj/marked
https://github.com/chjj/marked

デザインをGithub風にした。
sindresorhus/github-markdown-css
https://github.com/sindresorhus/github-markdown-css

fix #257, #381, #382
Merge pull request #384 from kmiyake/iss-257_implement_preview_feature_on_issue_form

Implement preview feature for creating and editing issue