Add logging options

I've added the ability to define log formats inside the main instance configuration and use one of the formats for the global access log. I also added a flag under the vhosts to enable, disable or leave undefined the access logging behaviour.

The tri-state behaviour might be a little confusing, but the idea is that that access logs can be disabled for a vhost by passing `false` or left unspecified by passing `undef`; this tells nginx to use the `access_log` directive from the main configuration. The default is `true`, which keeps the current behaviour of generating a log filename based upon the domain.