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

[Mpdreamz]

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 property names but not whether a property should be ignored when serializing (i.e when calling Index<T>(doc)) or when calling .MapFromAttributes() inside .Map<T>(). As reported by @mausch in #1069

.MapPropertiesFor<T>(props => props
    .Add(p => p.MyProperty, PropertyMapping.Ignored)
    .Add(p => p.SomeProperty, "someprop")
)

[gmarz]

@Mpdreamz This change looks good to me, but one slight nitpick/suggestion...

I wonder if this would be a more intuitive interface:

.MapPropertiesFor<T>(props => props
    .Ignore(p => p.MyProperty)
    .Add(p => p.SomeProperty, "someprop")
)

This way one doesn't have to know about the static Ignored instance of PropertyMapping.

[Mpdreamz]

Excellent suggestion @gmarz

Updated so it uses a dedicated descriptor instead of a fluentdict:

.MapPropertiesFor<T>(props => props
    .Ignore(p => p.MyProperty)
    .Rename(p => p.SomeProperty, "someprop")
)