Add support for serveraliases in vhosts

These vhost shorthands don't currently support setting serveraliases, which the
underlying nginx::vhost does. Allow use of serveraliases in vhost::* by adding a
default parameter to each.
Merge branch 'vhost-serveraliases'
Remove insanely-formatted *_proxy.log
Send X-Forwarded-{Proto,Server,Host} for proxies
Make Host header forwarding configurable

Sometimes you don't want to forward the requested Host header to the upstream
server. Add a `forward_host_header` flag to enable configuration of this
behaviour.