Support of LessThan and GreaterThan for strings

New release with support of Support of LessThan and GreaterThan for strings.

[TestMethod]
public void TestLessAndGreaterThan()
{
    var context = GetCoreContext(nameof(TestWhereAnd)); //EF Core     
    //or new BlogContext(testConnectionString);     in EF
    SeedForTests(context);

    var query = context.Authors.Query(q => q.LessThan("FirstName", "Mario"));            
    var first = query.FirstOrDefault();
    Assert.AreEqual(first?.FirstName, "David");

    query = context.Authors.Query(q => q.GreaterThan("FirstName", "Mario"));
    first = query.FirstOrDefault();
    Assert.AreEqual(first?.FirstName, "Some");
}

Thank you @boris612 for this nice collaboration.

For more information look at the pull request: #8