Fix for Issue #1266

[Falven]

Added successfulbuild and failedbuild External commands; modified postbuild to run after every build.

[hlovdal]

This pull request for #1266 is a bit out of date. I have rebased the branch, cleaned up a bit and added changelog entries at the branch issue_1266 in my fork.

To update this pull request, run the following git commands

$ cd /your/repo/fork/directory
$ git remote add hlovdal https://github.com/hlovdal/vscode-arduino
$ git remote add microsoft https://github.com/microsoft/vscode-arduino    # If not already done
$ git fetch --all
$ git branch issue_1266 hlovdal/issue_1266
$ git push origin issue_1266
$ git remote remove hlovdal                # Optional

and change the target branch to microsoft:dev and the source branch to Falven:issue_1266.

[lersi]

hello, I created a community fork at https://github.com/vscode-arduino/vscode-arduino.
You are more than welcome to merge your changes there.