Adds LoggingFilterSwitch

[nblumhardt]

Runtime filtering control.

var @switch = new LoggingFilterSwitch();
var sink = new CollectingSink();

var log = new LoggerConfiguration()
    .Filter.ControlledBy(@switch)
    .WriteTo.Sink(sink)
    .CreateLogger();

// Written
log.Information("Adding {Volume} L", 11);

@switch.Expression = "Volume > 12";

// Not written
log.Information("Adding {Volume} L", 11);

[nblumhardt]

Hey folks; a second set of eyes on this would be great, but since we're in pre-release mode I'll push it through. Cheers!