Add non array based WriteTo support

[JonruAlveus]

Conversation started here but new issue to discuss a solution.

Is your feature request related to a problem
Currently WriteTo uses an array syntax but this makes it very hard to override the configuration using other sources (such as EnvironmentVariables).

Describe the solution you’d like
A way to specify the WriteTo section without a top level array, as suggested by @nblumhardt:
WriteTo": { "File": { "path": "log1.txt" }, "RollingFile": [ { "pathFormat": "log2.txt" }, { "pathFormat": "log3.txt" } ] }

Additional issues
How to handle sinks which have no parameters. : {}? : null?