key parameter for Cache tag

[duncanmcclean]

This pull request adds the ability for a developer to specify the cache key they want to use when using Statamic's {{ cache }} tag.

Instead of Statamic creating a random cache key, a developer can specify it with the key param.

{{ cache key="the_alphabet" }}
a b c d e f g h i j k l m n o p q r s t u v w x y z
{{ /cache }}

This PR implements statamic/ideas#349.

[jasonvarga]

I know this is exactly what that issue described, but I wonder if or how we should handle the scope parameter along with this.

If you were to have {{ cache key="something" scope="page" }}... should it work? If so, how would you flush it?
Or should we just say if you use the key param, you're on your own. The scope param has no effect.

[jasonvarga]

We decided what you're doing is great. 👍