[WIP] initial implementation of skipping prefix hash

[wiibaa]

As discussed in #25, when storing metrics into a hash, one could prefer to strip the name of the "container" from the metrics name outputting foo.bar to graphite instead of mymetrics.foo.bar
when the event contains a field mymetrics of type Hash

This PR propose to add a boolean config to strip this prefix and help is wanted in finding the best name for it as it is only applicable to hash-fields.

I'm still working on the doc and the spec, but would like feedback for the field name
@jsvd @purbon @jordansissel want to propose something better than skip_prefix_for_metrics_hash ?

[elasticsearch-release]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run; then say 'jenkins, test it'.