[Feature Proposal] Resolve function keys recursively

Problem

It would be useful to be able to reference values from `config.theme` no matter how many levels deep they are referenced from. Below are some use cases: When using a custom plugin to text styles which are a group of CSS declarations grouped together. [code block] Or creating a plugin which allows you to apply colour themes but you would like to reference which is the default. [code block] Or simply when trying to alias values in other situations. I had a look in the source code and I can see that `resolveFunctionKeys` in https://github.com/tailwindcss/tailwindcss/blob/master/src/util/resolveConfig.js is responsible for dealing with the above and can see it only looks at the first property key-value pair. I had a play around with it but I couldn't get the object to return correctly and I wasn't sure how to do it recursively, I could only do it a set number of levels. I think this would be a valuable addition if possible. What are your thoughts?