Add ignoreMutableCollections option to prefer-readonly-type rule

[taylorc93]

Hello! I'm currently migrating the main frontend repo for my company from a tslint / eslint hybrid to pure eslint. In our old config, we were using tslint-immutable's readonly-keyword to enforce use of readonly for our interfaces. However, we never enabled the readonly-array rule. Now that we're migrating to eslint-plugin-functional, we're getting a huge number of linting errors from the prefer-readonly-type rule since it appears to be a union of these two rules from tslint-immutable. While some of these are auto-fixable, many are not and fixing them manually would be too large of an undertaking for us right now.

To remedy this, I propose adding an ignoreMutableCollections option to this rule that allows users to disable checks against Arrays, Tuples, Maps, and Sets. This will make it much easier for users like us to migrate off of tslint and onto a more actively supported linting architecture. Happy to clarify the use case / expand on this PR as needed!

[RebeccaStevens]

Hey, thanks for the PR, I like it and can get behind it.
I'd just like to request one change before I merge this in. Could you change the name of the option from "ignoreMutableCollections" to simply "ignoreCollections"?

[RebeccaStevens]

Also, I should let you know that we are thinking about reworking this rule in future when I get some free time (see #153 for details).

[taylorc93]

That name change sounds good to me, I changed it in my latest commit. Thanks for the heads up on the upcoming change, I'll keep an eye on it in the background!

[RebeccaStevens]

Released in version 3.1.0