Add basic tslint.autoFixOnSave support (#2)

[kondi]

This pull request replicates the basic boolean version of tslint.autoFixOnSave config option of vscode-tslint extension.

Quite ugly solution which finds the code action having the exact 'Fix all auto-fixable tslint failures' title. As I understand the current API does not allow a more sophisticated solution, but I am open for implementing other proposal if you have one. At least this one is working now. I have opened a PR in microsoft/typescript-tslint-plugin#46, if accepted, there will be a nicer solution and will be possible to implement the array version of tslint.autoFixOnSave option too.

Related issues:

• Explore supporting "fix on save" #2
• https://github.com/Microsoft/vscode-tslint/issues/417

[msftclas]

All CLA requirements met.

[mjbvz]

Change looks good overall. Thanks for your work!

However I'd use the Source.fixAll purposed VS Code api instead of onWillSaveTextDocument for a few reasons:

• Fix all actions can be triggered without saves.

• Users can setup keybindings for fix all actions

• We'd have build-in VS Code setting for this instead of having to provide our own:

  "editor.codeActionsOnSave": {
  	"source.autoFix.tslint": true
  }

• codeActionsOnSave runs in a more standard order relative to other on save actions

The downside of fixAll is that it will only work properly in VS Code 1.31+ but I think the trade offs are worth it

I'm going to merge this PR in as-is and just refactor the onWillSaveTextDocument to use Source.fixAll instead

[MathiasKandelborg]

Came here to say how happy I am this is fixed! Note to self: I'll be sure to remember to report bugs & issues on VSCode extensions in the future