Add support for setup and teardown

[evanmcclure]

Add support for setup and teardown.

[yugui]

What use case do you have?

The lazy-evaluation nature of Jsonnet does not evaluate an expression unless it is directly or indirectly referred by the final output. The purely-functional nature of Jsonnet does not usually allow us to have side-effects.

It means that you just need to refer the expression when you want to guarantee it is evaluated, and that you don't have any side-effects that you have to cleanup in "teardown".