Add datetime comparisons for filtering (#93)

[rbramley]

Issue #93 (plus others like #223) requires the ability to perform filtering with simple date comparisons.

This patch determines whether a type is a Neo4j Date/Time and augments it with filters for GT/GTE/LT/LTE (over and above the existing augmentation for a Neo4j type).

[Andy2003]

Currently we are trying to harmonize our API with that of the official JavaScipt version (https://github.com/neo4j/graphql).
In this sense we should replace the (input-) types like _Neo4jDateTime etc. by simple scalars which transfer the date / time as iso string.
With this in place you would pass a complete date / time iso string in the filter.

see also:

https://github.com/neo4j/graphql/blob/master/packages/graphql/tests/tck/tck-test-files/schema/types/datetime.md
https://github.com/neo4j/graphql/blob/master/packages/graphql/tests/tck/tck-test-files/cypher/types/datetime.md

This means we need some refactoring:

• configuration to switch behavior until next major release
• implementation of the new scalars
• adjusting of the generated cypher

[Andy2003]

The adjustments mentioned in #229 (comment) are implemented via #235