Allow properties to be ignored in code when serializing/calling mapattributesfor()

There are two aspects that can only be configured using attributes when sometime configuring using regular code is the only way or simply easier. As per #1062 we could already override but not whether a property should be ignored when serializing (i.e when calling `Index<T>(doc)`). As reported by @mausch in #1069 

```csharp
.MapPropertiesFor<T>(props => props
	.Add(p => p.MyProperty, PropertyMapping.Ignored)
	.Add(p => p.SomeProperty, "someprop")
)
```