Remove dependency on DataFrames while preserving functionality

DataFrames takes _way too long_ to load and isn't needed for the the majority of Mustache's functionality, so I adjusted the context lookup functions to use DataFrames only if it's been loaded by the user and removed the dependency on and loading of DataFrames by Mustache.

In short, everything still works and it loads much faster if you don't use DataFrames.

I also fixed a couple bugs in the tokenizing and tests.