Allow to use on fields with existing data

```
Attempt to update field Content failed: field_sql_storage cannot change the schema for an existing field with data..
```

Currently I have to alter field tables myself like this:

``` sql
ALTER TABLE field_data_field_content ADD field_content_view_mode VARCHAR(32);
ALTER TABLE field_revision_field_content ADD field_content_view_mode VARCHAR(32);
```
