- Remove :string_method option.
- Add #tags_string_for(context) instance method.
- Throw error if either :field or :string_method options are set; these have been removed as of version 1.1.0.

The reason for removing string method is that we automatically define "tags_tagged_with", "tags_separator", etc. with hardcoded names.

If the user wants a custom string method, they can do either of the following:

```ruby
def my_string_method
  tag_string_for(artists)
end
```

OR

```ruby
def my_string_method
  artists_string
end
```