Allow configuration of 'discouraged_object_literal' rule

[shagedorn]

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New rule request

Similar to the object_literal rule, I'd like to have configurability for the inverse discouraged_object_literal rule, to enforce literals for some objects, and enforce the opposite for others.

Currently, only enforcing literals can be configured, but you cannot selectively discourage the others.

The main motivation comes from Xcode 10's terrible image literal support (you can basically only use them by dragging from the media library, and it's very hard to see or edit the raw image names in Xcode, nor do you see a preview).

Our goal is to enforce literals for all objects (currently that means colors), except for images, where we want to enforce non-literal syntax.

The relevant part of our config file would look like this:

opt_in_rules:
  - discouraged_object_literal
  - object_literal

# this part doesn't work yet
discouraged_object_literal:
  color_literal: false

# this already works
object_literal:
  image_literal: false

I would assume the existing ObjectLiteralConfiguration can be used for both rules.

Unless I'm missing something and this option shouldn't be implemented after all, I would volunteer to look into the code myself.

[marcelofabri]

A PR would be welcome!

[shagedorn]

Thank you 🎄