Add target_concat plugin

Hello  !

We made a new target to merge/concat plugin instances.
With this : 
```
<Chain "PreCache">
  <Rule "merge_cpu" >
    <Match "regex">
      Plugin "^cpu$"
      TypeInstance "idle|system"
    </Match>
    <Target "concat">
    </Target>
    Target return
  </Rule>
  Target stop
</Chain>
```

You can transform this :
1383922533.751 tcohen-pc.mtl.sfl/cpu-0/cpu-idle value=6.999575e+01
1383922533.751 tcohen-pc.mtl.sfl/cpu-0/cpu-system value=4.999696e+00
1383922533.751 tcohen-pc.mtl.sfl/cpu-1/cpu-idle value=9.299431e+01
1383922533.751 tcohen-pc.mtl.sfl/cpu-1/cpu-system value=0.000000e+00

to this : 
1383922533.751 tcohen-pc.mtl.sfl/cpu/cpu-0-idle value=6.999575e+01
1383922533.751 tcohen-pc.mtl.sfl/cpu/cpu-0-system value=4.999696e+00
1383922533.751 tcohen-pc.mtl.sfl/cpu/cpu-1-idle value=9.299431e+01
1383922533.751 tcohen-pc.mtl.sfl/cpu/cpu-1-system value=0.000000e+00