Expose `IgnoreColumn` boolean on `ColumnInfo`

By doing so it makes the fluent mappings much cleaner

```c#
s.Columns.IgnoreWhere(mi => ColumnInfo.FromMemberInfo(mi).IgnoreColumn);
```

This also will set `IgnoreColumn` to true if `[ExplicitColumns]` is set on the class but the property does not have an explicit `[Column]` attribute.

Pull request is partially related to issue #67