Provide a way to add multiple content handlers

[naholyr]

We can have many helpers, hooks, etc… but only one content handler, this makes it quite uneasy to append new content handlers above existing ones. For example in my case I use:

• my dedicated custom content handler for some specific pages, which falls back to…
• a content handler generating RSS, itself using…
• blog-content-handler, which falls back to…
• default content handler

Maybe the content_handler(s?) option could be an array, and core would:

• unshift default content handler (so it's always here, cannot be disabled, or another option?)
• concatenate all their "getSections" & co
• for every negotiation: call all "negotiateContent", and merge provided contents (in the same order, last handler = most priority)

I don't think this would be too hard to do. What do you think? Do you see major issues with this vision (before I start working on PR)?