Plugin.php: Update duplicated code

There are some duplicated code when I commit the file before.
Remove them now.
Merge pull request #4 from weakish/patch-1

Plugin.php: remove duplicated code
Merge pull request #3 from mutoo/master

Add GoogleAnalytics Plugin and Upgrade LaTex Plugin
修复界面结构，改进拼写
阿里云存储插件
Merge pull request #7 from loftor-git/master

阿里云存储插件
Merge pull request #5 from fenbox/master

修复界面结构，改进拼写
GitHubGit: roughly compatible with jekyll

Use the same jekyll structures, so you can post on jekyll and typecho
at same time.

- Only watch files in `_posts`.
- Add markdown support.
- Support jekyll style YAML fronter.
Merge pull request #8 from weakish/master

支持Markdown和Jekyll的YAML头
Add MathJax plugin