Add an option for logging

[roblourens]

In the Chrome debugging extension, I've found the 'diagnosticLogging' option really useful. It logs basically everything that happens within the extension to a file, and is easy to upload to github when reporting issues. It might be useful to look at how we can provide better logging to diagnose issues faster.

The non-debug part of the extension can log to a channel in the output pane, like Typescript, eslint, tslint, etc. We could log errors there, with a verbose mode to log everything. The input and output of all the external tools we use would be useful.

[ramya-rao-a]

The debug part is done with 0ea5206

[hyangah]

It's a great idea and we will follow up in golang/vscode-go#54