Added support for appsettings configuration

[nrandell]

PR for issue #5 that adds in functionality to use the appsettings in a single line

[nrandell]

I've removed the additional dependency to serilog.settings.configuration, and created a logger builder extension to add serilog. This keeps the original package nice and clean.
The example now uses

.UseSerilog(configuration =>
    new Serilog.LoggerConfiguration()
        .ReadFrom.Configuration(configuration)
        .CreateLogger()
)

[nblumhardt]

Thanks for pushing ahead with this one, @nrandell 👍

I think this has become effectively a duplicate of #1, where there's already been a bit more discussion around trade-offs. I think we should close this PR (sorry!) and pick things up over there. Sound like a plan?

[nrandell]

I've just read #1 a bit more carefully @nblumhardt and it makes sense. So I'm happy to close this - it's good to see that a number of us have come to the same conclusions separately! We must be doing something right!