Automatically show output panel

[oNaiPs]

Implements spec from #104.

Allows user to specify, per command, wether they want to show the output panel or not.

Adds the following new setting:

- ...
- `commands`
  - `autoShowOutputPanel` - Automatically shows the output panel:
    - `never` - Never changes the output panel visibility (default).
    - `always` - Shows output panel when the first command starts.
    - `error` - Shows output panel when a command fails.
    - ...

Example usage on vscode settings.json:

  "emeraldwalk.runonsave": {
    "commands": [
      {
        "cmd": "pwd",
        "autoShowOutputPanel": "always"
      }
    ]
  }

[bmingles]

Looks good overall. I think we need to get rid of the auto-hide feature (see inline comment) and update docs accordingly.

Also, I know I don't have any instructions on PR format yet, but it would be good to have an example settings in the PR description that I can test the functionality with.

[oNaiPs]

@bmingles should be done now, let me know if that works.

[bmingles]

LGTM. Thanks for the PR.

[bmingles]

@oNaiPs Your change should be available in the 0.3.2 pre-release:
https://github.com/emeraldwalk/vscode-runonsave/releases/tag/v0.3.2-pre