Clean up extensions a bit. Removes newest form of registering extensions, more consistently sets and uses an extension's ext_name, and makes a lot of things errors instead of just log messages in hopes that people can't get too far with a messed-up config.
Remove yet another way to register extensions and register/activate FrontMatter like a normal extension.
Fix some references to extensions[:frontmatter]
Merge pull request #1237 from bhollis/ext

Clean up extensions a bit