URL parameter placeholders in graph definitions

This lets us call something like
    ````http://graphite.example.net/system/common/details/cpu/?p[node]=somenode.example.net````

And refer to that node in the graph definitions like so
    ````:data  => "servers.%{node}.cpu*.cpu-wait.value"````

This PR inspired by tanelj/gdash@0b4560900.