Define extension story for tasks

[dbaeumer]

Out of the box VS Code ships with a set of predefined task templates. Since VS Code 1.9 we also support defining more than one command in the tasks.json. Hence the value of a template is not huge anymore since most users will use different commands in a tasks.json.

However extension like C# or C++ want to contribute to the tasks.json to ensure that a build tasks for example executes a C++ specific build script.

We need to define how extensions best participate in running builds and tests.

[dbaeumer]

The current plan to address this is to provide API so that extensions can provide tasks.