add variable support

[skewballfox]

In order to support the snippet syntax in editors without builtin snippet functionality, and also in order to support variables beyond those specified in the specification, variables (eventually) will have to be handled by sniper itself.

this also adds a much needed layer to the parsing logic. Right now the parsing is only handled on the first call of the snippet, modifying it so that future calls won't have to rebuild a snippet composed of snippets. But variable handling(along with static text transformations), will have to be handled per call.

[skewballfox]

This has been started with the Functor type component, I'll close this once the type is fully supported.