added jsdocs_autoadd_property_tag config item for variables

I was trying to think of a way to use the `extra_tags` setting to add tags for functions vs variables.  This would allow us to combine the jsdocs_autoadd_method_tag and new jsdocs_autoadd_property_tag config values.

Something like:
```javascript
"extra_tags": [
  {"type": "function", "tag": "@method ${1:%s}"},
  {"type": "variable", "tag": "@property ${1:%s}"}
]
```

But my knowledge of Python is too limited to actually implement this.