Add before_index proc so that it can be called every time index is hit

[boberetezeke]

I needed a hook so that I could create an oauth token based on the logged in user

[grape-bot]

	2 Warnings
⚠️	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#85](https://github.com/ruby-grape/grape-swagger-rails/pull/85): Add before_index proc so that it can be called every time index is hit - [@boberetezeke](https://github.com/boberetezeke).

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[dblock]

This needs doc, tests and a passing build please.