Custom nginx config "slots"

[dshoreman]

A leftover from #4 which was otherwise completed long ago. A project's apps should have some way of defining custom config "slots" which would get injected in/near the writeNginxConfig() method.

Since a project's config file will have multiple points at which additional rules could feasibly be injected, we'll probably need to have a list of "named slots". Essentially pre-defined variables ("placeholders") in the Blade template.

If we had a single "custom config" block, it'd end up being too restrictive and result in dodgy hacks to "fake" the end of a block just so that code can be injected after it.