Added intermediate pluggin extender (#1)

[JimmyBjorklund]

Extender (intermediate plugin)
This intermediate plugin extends IPFIX records by adding new items to the existing record set using user-defined expressions. It reuses the filter functionality to find out if the records are to be added or not if there is no match there will be a 0 or empty string added as default values.

Example configuration

<intermediate>
  <name>Extenders</name>
  <plugin>extender</plugin>
  <params>
    <expr>dot1qCustomerVlanId == 0</expr>
    <id>VRFname=default</id>
  </params>
</intermediate>

The above sample adds a new string field VRFname=default to each IPFIX record if the existing field dot1qCustomerVlanId equals zero.

Parameters
expr
The filter expression to evaluate.
id
The identifier string to be added to the record when the expression evaluates to true.