Add prgm support

This release adds prgm support. prgm will search the current directory for a file matching the provided name (case insensitive; extension of .8xp). If found, the file will be passed to the command ti842py {filename} --run, where {filename} is the name of the file. For example, prgmTEST would search for TEST.8Xp (case insensitive) in the current directory. If a path is given that contains . or ~, or if the path is not all uppercase, the transpiler will assume that the path has been manually provided by the user (this will be useful once I implement a TI-BASIC shell).