Add helm schema command

[carolynvs]

Prints out the json schema for the mixin's yaml input. We are trying this out (prototype only, not a full implementation) to see if this is sufficient for integrating with IDEs to get autocompletion for #6

@itowlson Can you take a look and let me know if this looks right? Then I can merge and you can try it out on the canary binary.

Example Output:

$ ~/.porter/mixins/helm/helm schema
{
  "install": {
    "type": "object",
    "properties": {
      "name": {
        "type": "string"
      },
      "chart": {
        "type": "string"
      }
    },
    "additionalProperties": true
  }
}