Add advanced comparisons to custom attribute filter

[paynejd]

#39 is focused on adding the ability to filter resources when a specific custom attribute is "equal to" a specified value. We want to add support for additional types of comparisons, such as "contains", "begins with", "ends with", "not equal", etc. Step 1 is to prepare the list of comparisons/filters that are possible and the approximate level of effort for implementing each.