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.
instead of this, how about doing something like
```ruby
@resource_limits = Template.default_resource_limits.merge(resource_limits || {})
```
so that you can override single values without losing the other defaults?