Building should clear the previous build log

[sparvanders]

To look at the error reports of a build, I need to read from the beginning to follow the build process from the start. However, old build logs are not removed automatically. Instead the new build is just appended. This means I can't scroll to the beginning of the terminal buffer to find the start, unless I first manually press the "kill terminal" trashcan icon. This is a manual step that I might forget to do, and that can confuse new developers. (It took me a good while of debugging before I discovered this behavior!)

Eclipse works the expected way, removing the old log.

[ivankravets]

You control VSCode task runner. See http://docs.platformio.org/en/latest/ide/vscode.html#task-runner