Allow admin to configure where templates, static, pages, and keys directories are. Requested by SamWhited in #63.
This will come in handy for packaging the application for various Linux distros, and for WriteFreely.host.
Add new fields under config.ServerCfg:
If any of these are blank, default to the current directory.
For the foreseeable future, it's fine to not include these values as part of the interactive configuration process.
TODO: Note in the documentation that a server restart is required when changing these values.