Implemented freevars

Arbitrary variable tokens can now be defined (though at this point only in the code itself) as a pair of a variable name and a filename pointing to a plain text file containing a newline-delimited list of elements. When a token of the form $name$ (where name is the name of a variable) is encountered, the output will include a random element from the appropriate list. The variables $name$ and $noun$ are hard-coded at this point, but the program will not crash if names.txt and nouns.txt do not exist and will instead just silently ignore the variables.