Make it easy to set default resource limits.

@fw42 & @camilo
cc @grollest & @sunblaze 

## Problem

Right now Shopify is monkey patching Liquid::Template#render in order to set default resource limits.  This seems quite brittle and could be simplified for us and others by providing a default_resource_limits hash.

## Solution

Add a Liquid::Template.default_resource_limits hash that gets used by when resource limits aren't explicitly provided to the context.
I love the idea of using `.merge`.

However, I don't see why render_score_current or assign_score_current should be allowed to be modifiable.  I also don't want to remove the `.dup` in Template#initialize since that would mean that changes to the template instance's resource limits would actually affect the defaults.