Implement strategic merge patch

[vassilvk]

Add merge op to the patch section of a ModRule.

This operation should trigger a Kubernetes strategic merge patch as implemented here: https://github.com/kubernetes/apimachinery/blob/master/pkg/util/strategicpatch/patch.go#L856.

The path expression of a merge patch operation determines the "original" JSON which is to be patched by the strategic merge.
The value of the patch is considered to be the "patch" JSON object passed to the strategic merge processing.

All the normal rules of select apply here - if select is present and it yields multiple results, then multiple merge operations will be performed, each time evaluating path and value in the context of the next selected item.

Example:

...
  patch:
    - op: merge
      select: '$.spec.template.spec.containers[? @.name == "my-container"]
      path: '/spec/template/spec/containers/#0/env'
      value: |-
        - name: ENV2
          value: true

The above patch will find container named my-container and it will inject environment variable ENV2 into it, without disturbing the remaining environment variables of the container.

This feature will solve #77 .