Support multiple IBlogService implementations or abstract the providers of blog and comment entities

[Siderite]

When you migrate a blog to miniblog, you usually have some sort of backup. For example I had the posts in a Blogger.xml file and the newer comments in a Disquus export. In order to not have to change the source of Miniblog, I would like to have the option to read blogs and comments from the original backups, using a custom implementation that I inject, but when edited they get saved in the main format.