Support the ability to specify a default template

I wanted to avoid putting the same `{template: '...'}` in all my markdown files, so I changed it so that you can specify a template in the default environment and it will use it. It also stops the use of a template on the template file itself (solving most of the circular reference situations). Updated the tests for this change.

Also worth noting that your fixture test fails with `marked@0.2.10` as they added header IDs. Didn't want to update the test because it will fail for v0.2.9. You should probably specify the exact version of `marked` to use.