A more robust version of stream_tweetsSource:
Name of directory in which json files should be written. The default, NULL, will create a timestamped "stream" folder in the current working directory. If a dir name is provided that does not already exist, one will be created.
Logical indicating whether to append or overwrite file_name if the file already exists. Defaults to FALSE, meaning this function will overwrite the preexisting file_name (in other words, it will delete any old file with the same name as file_name) meaning the data will be added as new lines to file if pre-existing.