Add 'edit tags/categories' functionality to post-editor

[TylerYang]

It should work in this way,

1. Double click item in the post list then popups a dialog(Better use a small one.)
2. In the dialog, user should be able to edit(delete, create, add) tags/categories.
3. Click _CONFIRM_ or _OK_ button, send a request to server(hexo-pl), get the response, parse it => show the message => Done.

Things need to think over,

1. Maybe not to use double click, because it is not so easy to know, also it is not a good way to double click the item in
2. Maybe it is better to show the auto complete list when user enter the characters. (Need to make a PR to hexo-pl)