Implement data source insert convenience methods

[jessesquires]

In #67 we added some convenience methods to DataSource to remove items.

Let's add similar methods to insert.

https://github.com/jessesquires/JSQDataSourcesKit/blob/develop/Source/DataSource.swift#L166-L189

Tasks:

• Add 2 new methods, one with (row, section) and one with IndexPath
• Add docs
• Add unit tests

[jessesquires]

cc @psartzetakis 😊

[psartzetakis]

I will take care of it 😄

[jessesquires]

@psartzetakis -- By the way, there's no pressure here! Don't feel like you have to do any of these new tasks if you don't want to. 😄

I just wanted to let you know about them. 😊

[psartzetakis]

Yeah sure, I am aware of that. 😊

[psartzetakis]

Hi @jessesquires ,

I have a question here. Let's assume that we have 2 sections (section-0, section-1).
If the user wants to add an item in section-2 which strategy do we follow?

1. We are not allow the creation of the new section so nothing happens? (kind of restrictive)
2. We create a new section if it doesn't exist. Which leads to the following question:

• if the users requests section-2 it kind of makes sense to add it, but what if he requests section-5? Do we ignore his preference for section-5 and we create the section-2 instead?

[jessesquires]

@psartzetakis

Let's mimic the Array APIs, you can't insert out-of-bounds. That will keep it simple for now. 👍