Feature request: reference instances by env+type

[ghost]

Often I have only one server per env+type combination (i.e. production+database). It'd be nice to say things like rake servers:ssh[production,database] instead of the tedious dance of rake servers:list[production] + find the right id + rake servers:ssh[the-right-id] + triple-check I got the right id, right environment and right type + finally press enter.