Make AzureTableStorage sink work with AppSettings.

Adds an overload to the AzureTableStorage extension
method that takes a connection string rather than just
the storage object. This allows configuration similar
to the following to just work:

    <add key="serilog:using" value="Serilog.Sinks.AzureTableStorage" />
    <add key="serilog:write-to:AzureTableStorage.connectionString" value="UseDevelopmentStorage=true;" />