Added ignore callback support

Hi Steve,

I am loving knockout so far and find the knock mapping plug-in very useful.
But I missed a generic way to ignore properties to map (currently, you can ignore them by name only).

Hence I decided to add an optional ignore callback to the option - allowing  to ignore properties based on patterns (e.g. some naming convention) additionally to the ignore array.
I am using this as a global configuration and all my models (if they follow the naming convention) work with that just fine.

I hope that is a useful addition to have and wanted to contribute this (this is the first time I do such a thing, so apologies for any issues I am causing).

If my approach is not acceptable though, it would be nice to see some similar approach (probably to the other configuration types as well) in the future.

Thanks

Michael