feat(experimental): begin support for text content whitespace handling

[aimee-gm]

Fixes # 51.

Checklist

• Added validaton to any changes in the parser API.
• Added tests covering the parsing behaviour changes.
• Linked to any relevant issues this will close.
• Tested the output using the demo.

Changes to parsing behaviour

With an experimental flag, whitespace will be handled in textContent()

Example input covered by new behaviour

<div class="h-entry">
  <span class="p-location h-adr">
    <span class="p-locality">Berlin</span>,
    <span class="p-region">Berlin</span>,
    <span class="p-country-name">DE</span>
    <data class="p-latitude" value="52.518606"></data>
    <data class="p-longitude" value="13.376127"></data>
  </span>
</div>

Example output from new behaviour

{
  "items": [
    {
      "type": ["h-entry"],
      "properties": {
        "location": [
          {
            "type": ["h-adr"],
            "properties": {
              "locality": ["Berlin"],
              "region": ["Berlin"],
              "country-name": ["DE"],
              "latitude": ["52.518606"],
              "longitude": ["13.376127"]
            },
            "value": "Berlin, Berlin, DE"
          }
        ]
      }
    }
  ],
  "rels": {},
  "rel-urls": {}
}

Other changes

• Adds isEnabled() method for determining if an experimental option is enabled.