Output template support for {Properties}

[nblumhardt]

Via: http://stackoverflow.com/questions/38894310/serilog-saving-event-properties-added-with-forcontext-into-flat-files

When writing log files (and to the console), it's sometimes desirable to include various properties not formatted into the message, and not explicitly specified in the output template.

This might look like:

Log.Logger = new LoggerConfiguration()
    .WriteTo.Console(outputTemplate: "{Level:u2} {Message} {Properties}{NewLine}")
    .CreateLogger() ;

Log.ForContext("Foo", 42)
    .Information("Hello from {Bar}!", "bar");

// -> INF Hello from "bar"! {Foo: 42}

Note that the properties written should exclude those already formatted into the template; it's a text logging scenario, if a full property listing is desired one of the JSON formatters will cover that case.

Marked breaking because this will interfere will logs that already use a static {Properties} element in their output templates, though in practice I can't see this appearing in many places.

[nblumhardt]

(It's possible to hack this into OutputProperties today without a huge amount of overhead, but rewriting OutputProperties to work more efficiently would be a great general improvement - there are a few unnecessary allocations we could clean up in there.)