First pass at Markdown-only blog posts

Updates the blog plugin to be more configurable and work with pure(ish) Markdown files.

I discussed this via email with Koen a few months ago, but I haven't heard back in a while. Figured I'd go ahead and submit it and see what happens. I don't necessarily expect this to get merged right now, just want to put it out there for discussion and improvement. Here's my last email to Koen:

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You can see I had to make a couple of tiny changes to page.py. There are other ways around that problem but that was the least intrusive I could think of. If you'd rather solve it in a different way, let me know and I can adapt the blog plugin accordingly.

Here's a writeup of how it works right now: [https://gist.github.com/cz/5f8e58e76dfa9d0f8e3e]. Could be the basis for user docs later on.

Feel free to comment inline on the commit if there are changes you'd like to see. Or if this looks good enough for a pull request just let me know and I'll submit it.

Finally, was curious to know what your vision is for the plugin system in general. Is it as extensive as you'd like it to be? Do you imagine any sort of ecosystem of externally-maintained plugins? I'm inclined to continue tinkering with this (exploring stuff like tags and excerpts) to see if we can arrive at a proper Jekyll/Pelican alternative.