Configure client munin-way

http://munin.readthedocs.org/en/latest/plugin/use.html#configuring
I think that the package should respect munin's recommendations and use [env](http://nodejs.org/docs/v0.8.22/api/process.html#process_process_env) configuration instead of [this code]
(https://github.com/newleafdigital/nodejs-munin/blob/f88796/lib/munin_nodejs_#L57).

Then we could use munin plugin config like this one:
```
[munin_nodejs_*]
env.statsHost 127.0.0.1
env.statsPort 8080
env.statsPath /stats
```
And therefore let it know where it should pull data from.

PS: Please also let me know if you don't have time to support the package. I'll make this all available in my own fork.