Make the write call of StringTagField::saveInto method configurable

[dkliemsch]

With that PR its possible to opt-out of the immediate write call within the saveInto method of the StringTagField using a custom config like this:

---
Name: app-tagfieldconfig
After:
  - tagfieldconfig
---
SilverStripe\TagField\StringTagField:
  immediate_write_enabled: false

[dhensby]

I think we certainly need a test for this.

[dkliemsch]

I think we certainly need a test for this.

I've added two tests with the option enabled/disabled, hope that's ok?