Add Diagnostic logging level for protocol message JSON

[daviwil]

This change adds a new "Diagnostic" logging level which is used for
logging the JSON contents of protocol messages that are read or written.
This will provide two benefits:

1. Increase the privacy of user logs that are transmitted over GitHub
2. Increase the performance of writing Verbose logs

We still log the high-level message details like message type, method,
and ID so that we can keep track of the message sequence.

[daviwil]

Example of new Verbose logs for messages:

6/15/2017 11:38:04 AM [VERBOSE] - Method "ReadMessage" at line 123 of C:\dev\PowerShellEditorServices\src\PowerShellEditorServices.Protocol\MessageProtocol\MessageReader.cs

    Received Request 'initialize' with id 0

6/15/2017 11:38:04 AM [VERBOSE] - Method "WriteMessage" at line 61 of C:\dev\PowerShellEditorServices\src\PowerShellEditorServices.Protocol\MessageProtocol\MessageWriter.cs

    Writing Response 'initialize' with id 0

6/15/2017 11:38:04 AM [VERBOSE] - Method "ReadMessage" at line 123 of C:\dev\PowerShellEditorServices\src\PowerShellEditorServices.Protocol\MessageProtocol\MessageReader.cs

    Received Event 'initialized'

6/15/2017 11:38:04 AM [ERROR] - Method "DispatchMessage" at line 147 of C:\dev\PowerShellEditorServices\src\PowerShellEditorServices.Protocol\MessageProtocol\MessageDispatcher.cs

    MessageDispatcher: No handler registered for Event type 'initialized'

6/15/2017 11:38:04 AM [VERBOSE] - Method "ReadMessage" at line 123 of C:\dev\PowerShellEditorServices\src\PowerShellEditorServices.Protocol\MessageProtocol\MessageReader.cs

    Received Event '$/setTraceNotification'