Adds optional performance support for apps using an immutable data structure such as redux. Adds the `immutable` boolean option for compile and an `immutable` option to store as well. When these options are used, computed will not recompute if the object has not changed. If your data structure is not immutable you should not use this as svelte cannot know if a mutation was made on objects.
This PR also adds support for Dates and NaN values so computed properties will not recompute if a date has not changed or a value did not change from NaN.
This closes out these issues:
* https://github.com/sveltejs/svelte/issues/1146
* https://github.com/sveltejs/svelte/issues/1161
This is my first PR for Svelte. Any feedback would be appreciated!
