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:

• map
• sort
• sort_natural
• sum
• uniq
• where
• 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.