JSON Document to allow for validation of datasource parameters

[springmeyer]

Mapnik-reference could provide additional data about what exact parameters mapnik datasources accept and require. This way calling applications could more easily validate datasources via their parameters without having to instantiate the actual mapnik objects (which may throw if resources are not available or which might incur unnecessary overhead).

A document could look like:

{
  "variables": {
    "encoding": {
      "type": "string",
      "doc": "The encoding value for the datasource attributes",
      "default-value": "utf-8",
      "default-meaning": "UTF8 will be the assumed encoding for string attribute values",
      "required": false
    },
    "row_limit": {
      "type": "integer",
      "doc": "Max amount of features to read from the datasource",
      "default-value": "0",
      "default-meaning": "All features will be read from the datasource (unless row_limit is > 1)",
      "required": false
    },
    "file": {
      "type": "string",
      "doc": "Path to the file for the datasource",
      "default-value": "",
      "default-meaning": "",
      "required": true
    }
  },
  "datasources": {
    "csv": {
      "file":"<@(file)",
      "row_limit":"<@(row_limit)"
    }
  }
}

[springmeyer]

underway over at https://github.com/mapnik/mapnik-reference/blob/master/latest/datasources.json and mapnik/mapnik-reference#44