Issue Type: Feature Request

The integrated terminal split panes should support launching a task from the command palette into a split terminal panel.

Here is the current workflow

1. Launch a long-running task from command palette by typing ctrl+p -> task first-watcher-task -> enter. (A new terminal window appears, running the task).
2. Click the split terminal button to create a split terminal.
3. In the split terminal, type the arguments for the second task and run it by pressing the enter key in the terminal. Can't run ctrl+p -> task second-watcher-task -> enter because that would open a whole new terminal section of its own.

Here is the desired workflow

1. Launch a long-running task from command palette by typing ctrl+p -> task some-watcher-task -> enter. (A new terminal window appears, running the task).
2. Launch the second long-running task from comand palette by typing ctrl+p -> task second-watcher-task -> enter. Somehow, this task runs in a second panel of the split terminal, based on some extra command, a task configuration setting, or perhaps by having focus in an already opened split terminal.

VS Code version: Code - Insiders 1.22.0-insider (8133505, 2018-04-03T01:09:54.914Z)
OS version: Windows_NT x64 6.1.7601