Allow Param Reassign for Props

Currently, [`no-param-reassign`](http://eslint.org/docs/rules/no-param-reassign) is set to not allow reassigning of properties. This is creating lots of ESLint exceptions in our code as there are quite a few things we use that expect you to be able to do this.

Thoughts on changing this to disallow reassigning the param itself, but allow reassign for props on it? FWIW, this is the default
