add handlebars resolve hostname helper

[ssalinas]

hostname -> first resolved ip or host:port -> ip:port , e.g.

{{#resolveHostname upstream}}

[ssalinas]

/fyi @tpetr cleaned up the helpers. Also, apparently to call it without the end bracket, need to cal it as {{{resolveHostname hostname}}} vs {{#resolveHostname hostname}}. Any UnknownHostExceptions will bubble back up and cause the request to fail as expected.