Serilog.Extras.LambdaIgnore

Hi,

I have taken some inspiration (and code) from Serilog.Extras.Attributed, and created a Serilog.Extras.LambdaIgnore. When using attributed it is up to the poco being destructured to define what is being ignored. I need to change what properties are ignored depending on which system/service is doing the logging (and what sinks they are using). So I thought it would be useful to be able specify what properties are ignored using Lambda expressions during logger configuration.

Some things to be aware of:
- Only one type is supported per call to "UsingLambdaIgnores"
- Multiple expressions are supported per type. 
- The code that extracts the property name from the expression could be made more robust, and trace with SelfLog.
- Some code that is now common between Attributed and this extra could be consolidated.
- The name could be improved.

So there are probably improvements to be made but I thought I would send the PR to enquire whether you think this adds enough value to include as an extra in Serilog.