Add support for VS Code's "code action" feature

[daviwil]

VS Code has a feature called "code actions" which allows a diagnostic marker (such as a Script Analyzer rule violation) to return a "code action" which can be performed to resolve it. There is currently prototyping underway to add suggested code corrections to some Script Analyzer rules so it should be fairly straightforward to wire this all the way through to VS Code.

I've started on an early draft of this integration, it's in a branch named daviwil/code-actions.

[daviwil]

This issue depends on the following pull request against PSScriptAnalyzer: PowerShell/PSScriptAnalyzer#508. I'll help get that PR driven to completion this week so that we can get this feature included in 0.6.0.

cc @kapilmb

[daviwil]

This feature was finished by @kapilmb and merged recently! #304