Support arbitrary expressions in functions, filters, and cluster aggregators

[lucaswoj]

Let's continue the conversation from mapbox/mapbox-gl-function#28 here 😄

To summarize so far:

• Arbitrary expression functions will make our filters, functions, and cluster aggregators more powerful & flexible
• Implementing arbitrary expressions will be challenging because they may need to run on the GPU
• We are leaning towards a lispy syntax comprised of nested arrays

[1ec5]

Also, devising a workable GUI for arbitrary expressions may be a challenge.

[lucaswoj]

My mistake, #47 is a better place for this conversation.

I moved over your comment @1ec5 😄