graphite-output: Added graphite plaintext proto formatting plugin.

graphite-output, a new value-pairs based template function is introduced.
It converts the incoming message to the graphite plaintext protocol format.
It is ideal to use with the monitor-source plugin generated messages.
Example usage:

destination d_graphite {
  tcp( host("localhost") port(2003) template("$(graphite-output --key monitor.*)"));
};

Signed-off-by: Tusa Viktor <tusavik@gmail.com>