Feature request: implement comparism for Nullable DateTimes and extended methods

[verbraucher]

Hello again, thank you for this plugin and the great support for the last feature!

It would be great to have some new features for filtering ranges of dates in the upcoming version:

• support comparism on Nullable DateTimes (Nullable< DateTime >) (this will throw currently System.ArgumentException : Expression of type 'System.Nullable[System.DateTime]' cannot be used for parameter of type 'System.Object' of method 'Boolean Equals(System.Object)'

• DateTime filtering for lower than (<), lower than equals (<=), greater than (>) and greater than equals (>=)

[Kusumoto]

Thank you for your feature request. I will explain your topic in issue.

• In feature support Nullable<DateTime> I can implement in next version.
• In feature support type DateTime in more filter condition (<,<=,>,>=) In case is impossible for implementation because generic function of Entity Framework is not support.

[crega]

@verbraucher I believe that this one is resolved with my merged PR