Add the `throwWhenMissing` option to the configuration

[acecilia]

This is a first look at the https://github.com/groue/GRMustache.swift/issues/69 issue.

With this PR it is possible to enable throwing when a box is empty, which means either the key is not in the context, or the key is in the context but its value is nil.

What is really my intention is to only throw when the key is not in the context, but I am having difficulties finding a way of telling when an empty box is due to the key not being in the context or its value being zero. I could use some direction here.

Is this something that you would like to merge?