Add on argument to add_nested

[hombit]

Feature request

Before submitting
Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

Following the semantics of the same keyword in:

• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.join.html
• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.merge.html

[hombit]

Implemented in v0.3.0