Provide Task Runner viewlet

<!-- Do you have a question? Please ask it on http://stackoverflow.com/questions/tagged/vscode --> We did a Task 2.0 deep dive today and this issue collects the combined feedback received. ### Task Properties - echo should be true by default so that users get a better idea of the task executed - we should support flattening the JSON properties as we do with other settings (e.g `"terminal.echo": true` instead of `"terminal": { "echo": true }` - we should better separate task declaration from task customization. Currently both happens in the tasks array property and the differentiating element is the `customize` property. ### Task Picker The task picker should support a decription / detail message to better explain what the task does. We also discussed whether the task picker should support MRU which would automatically move the most used tasks up to the top without customization. If we provide a task panel (see below) such a MRU list could be useful there as well. ### Task Customization The task customization is currently instance based where the task instance is identified by a task id, which can be quite 'cryptic'. An alternative way would be to identify a task based on a type and a key that is defined by the type and therefore can vary from task runner to task runner. So instead of having something like: ```json { "customize": "vscode.npm.run build", "taskName": "npm: run build" } ``` a customization would look like this ```json { "customize": { "type": "npm", "file": "${workspaceRoot}/package.json", "script": "build" }, "problemMatcher": ["$tsc"] } ``` where the structure of the identifying attribute is defined by the extension providing the runner support. In the example above the fact that a npm script is identified by the location of the package.json file and the script name comes from the npm extension. This might look different for task contributed by gulp of TypeScript ### Problem Matchers For task that execute a target of a task ru…