Allow the `custom_field_compare` to accept a value of `exists` or `not_exists`

Currently, the custom_field_compare query parameter only accepts the following values: “equal” (default), “not”, “before”, “before_inclusive”, “after”, “after_inclusive”.

If it’s possible to test for an empty string, we should expand this to allow a value of “exists” or “not_exists” to easily filter out posts where the meta is empty instead of needing to write custom_field_compare=”not” custom_field_value=””