Add `hexo migrate octopress <source>` command

Hi, Tommy:

It's a great pity for missing `hexo migrate octopress/jekyll` command, so I wrote this plugin to migrate octopress posts to hexo source and replace their filenames to standard hexo filenames (and also change 'categories' to 'tags') .

I imitated your coding style so that this plugin will not seem very strange in this repository.

finally, the command will be:

`hexo migrate octopress <source>`

the <source> field can be either a local directory or a remote github fetch url.