text: Title: Change features_only to feature_defs_only
  Body: When looking at the [DFS](https://featuretools.alteryx.com/en/stable/generated/featuretools.dfs.html#featuretools-dfs) call, I feel that the `features_only` option is misleading. Setting this to `True` only returns definitions and not the feature matrix. So I believe the option should be `feature_defs_only`
  
  #### Code Example
  
  ```python
  import featuretools as ft
  
  es = ft.demo.load_mock_customer(return_entityset=True)
  
  feature_defs = ft.dfs(
      entityset=es,
      target_dataframe_name="customers",
      agg_primitives=["mean"],
      trans_primitives=["time_since_previous"],
      features_defs_only=True,
  )
  
  feature_defs
  
  ```
  