Add support for slot _choices_ in upgrade card text

[spiegela]

There is only one example of this that I know of, but the "Heavy Scyk" Interceptor card allows for a choice of multiple upgrade slots. To support this, we could extend the format of grants in a couple different ways:

...
    "grants": [
      { "type": "oneOf",
        "values": [
          {
            "type": "slot",
            "name": "Cannon"
          },
          {
            "type": "slot",
            "name": "Torpedo"
          },
          {
            "type": "slot",
            "name": "Missile"
          }
        ]
      },
      {
        "type": "stats",
        "name": "hull",
        "value": 1
      }
    ]
...

Thoughts?

[jesseflorig]

I think this could benefit from my comment in #81 making it something like:

...
    "shipMods": {
        "chooseAddSlot": [
          "Cannon",
          "Torpedo",
          "Missile"
        ],
        "hull": 1
      },
    ]
...

[philtomblin]

I'd suggest something similar to Jesse's comment, which I think is more elegant, but taking account of my comment in #81. Like this:

{
    "name": "\"Heavy Scyk\" Interceptor",
    "id": 165,
    "slot": "Title",
    "points": 2,
    "ship": [
      "M3-A Interceptor"
    ],
    "text": "Your upgrade bar gains the [Cannon], [Torpedo] or [Missile] upgrade icon.<br /><br />Increase your hull value by 1.",
    "image": "upgrades/Title/heavy-scyk-interceptor.png",
    "xws": "heavyscykinterceptor",
    "grants": [
      {
        "type": "stats",
        "name": "hull",
        "value": 1
      },
      {
        "type": "slotChoice",
        "choice": [
          "Cannon",
          "Torpedo",
          "Missile"
        ],
        "value": 1
      }
    ]
  },

I'm happy to make the change if we can agree an approach...

[philtomblin]

Okay, I've submitted a pull request to address this issue.

Comments welcome...