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.
Ugh, using `@resource_limits = Template.default_resource_limits.merge(resource_limits || {})` actually breaks the tests like [test_resource_limits_hash_in_template_gets_updated_even_if_no_limits_are_set](https://github.com/Shopify/liquid/blob/4004cb63a5bfb6e76d4ae75f15f14ed041033b7e/test/integration/template_test.rb#L130-L136), since we allow resource limits to be accessed through the template.

I think what I have is good enough, since it only doesn't apply the defaults when creating a context explicitly and explicitly providing a resource limits hash.