Allow Token Create implementors to set any request params they would like.

[jonpugh]

I was implementing the "Hosted Link" feature in plaid and I couldn't do it with this library.

The params are hard coded.

This PR adds "$params" as an argument so that the caller can put any options in there they need.

To test:

1. Add to composer.json repositories:

        {
            "type": "vcs",
            "url": "https://github.com/allowance-app/plaid-sdk-php"
        }

2. Set require to use the branch:
   "tomorrow-ideas/plaid-sdk-php": "dev-feature/token-params",