Integrate with LoggerProviderCollection

[nblumhardt]

This will integrate UseSerilog() with the ILoggerProvider support in upstream Serilog.Extensions.Logging.

For the more self-contained (post-IoC-initialization) variant, it'll be as simple as UseSerilog(writeToProviders: true).

When logging is configured first, at program startup, a LoggerProviderCollection will need to be constructed and passed to WriteTo.Providers() as well as UseSerilog().

For some context, this will enable usage like: opentracing-contrib/csharp-netcore#42

Resolves #53

@andrewlock I'm hoping this will translate cleanly across to Serilog.Extensions.Hosting, and once we're happy with it here, I'll assemble a second PR :-)

• Working
• Tests Turns out there isn't really any (sanely) testable implementation in this package, it's all now in Serilog.Extensions.Logging
• Should the second UseSerilog() overload default to writing to providers? No :-)
• README updates
• Make sure providers added after logger initialization are still added to the LPC

[nblumhardt]

I know it's currently tricky for everyone to keep abreast of these changes; I think as in Serilog.Extensions.Logging our best bet is to send them through to dev and feel our way forwards from there.

Just one thing to call out - this changes the signature of UseSerilog() in a source, but not binary-compatible way. I think this is acceptable for this package, on a major version update, as it's unlikely to be involved in any nasty diamond dependency scenarios.

I'll hit the button in a day or so unless I hear from anyone.