Provide access to globals and filters

[mitsuhiko]

The discussion in #715 has triggered an abstract need to understand what's a known declaration in the template. Today you can look up variables by known name from the state, but you cannot enumerate them. The same is true for filters.

In theory something like State::declared_variables could be provided which is a list of all variables declared. Likewise State::filters could be added to return a list of filters that are registered by name. Providing access to the value itself seems out of scope for sure because filters are boxed internally and I do not want to expose these values.