Update nginx config to handle subdirectory installs

[ErisDS]

Our configuration of nginx will need to do extra trickery if the url of a blog is configured to use a subdirectory.

Also wanted to leave a note here about the idea of having nginx config template files easily accessible in the repo, and swapping template vars in, instead of programmatically building the file.

This improves readability/understandability of what ghost-cli does, and gives us a very simple logging fall back if we run into an issue where we can't write the config automatically.

E.g. if there's already a config file or the perms are wrong, we can log the template file and tell users to update their config to match this manually.