Simplified interface again and implemented post filters

• Removed the createPostItem from the client interface so that now you can actually use any React elements as PostItem, you don't have to pass the PostItem static variable anymore to PostList. A PostList is the only element needed by bloql to generate a list of posts. Ex: https://github.com/adriantoine/adriantoine.com/blob/7118ddcdc00f7fcb72b4e1a48363a460c5c9d03a/src/components/PostList.js
• You can now use filters: https://github.com/adriantoine/adriantoine.com/blob/7118ddcdc00f7fcb72b4e1a48363a460c5c9d03a/src/components/PostList.js#L9-L10