Added sortable property to column metadata

[rtsao]

It would be really helpful to have only certain columns be sortable. I'm thinking this should be a property in columnMetadata items:

[
  {
      "columnName": "name",
      "sortable": false,
      "displayName": "Name"
  },
  {
      "columnName": "location",
      "sortable": true,
      "displayName": "Location"
  },
  ...
]

[rtsao]

Fixes: https://github.com/DynamicTyped/Griddle/issues/60

[ryanlanciaux]

Thanks so much for adding this!! 👍

[tony-kerz]

awesome, should be added to documentation 👻

#307