Fix #56, but for real

[JesseTG]

Description

Extension methods for custom entity indexes weren't being applied to the generated Contexts class. This PR fixes that.

Fixes #56

How Has This Been Tested?

I ran the revised plugin on the sample code in Assets/ExampleContent and reviewed output in Contexts. Once I got it correct, I committed it alongside the code for the bugfix.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas Nothing to comment
• [x] I have made corresponding changes to the documentation Nothing to document
• [x] My changes generate no new warnings

[jeffcampbellmakesgames]

This has been merged to develop and should be available soon in v2.1.3.