Add deep search for filter taking in properties

[andershagbard]

This PR allows for backward compatibility deep search :woah:

Like so:

{{ input | where: 'some.nested.property', 'foo', deep: true }}
{{ input | where: 'some_nested_property', 'foo', deep: '_' }} # Custom separator

Added deep search for the following filters. This should be all Liquid filters which takes in a property argument:

• [x] map
• [x] sort
• [x] sort_natural
• [x] sum
• [x] uniq
• [x] where
• [x] compact

[andershagbard]

@dylanahsmith I am not sure if you still work on Liquid, but I would love for someone in the Liquid team to take a look at this PR.