Set different paths for input and output data

[reckoner165]

The current structure contains a single data directory that contains input .txt files as well as the output .wav. These should be moved to separate directories for clarity.

[onstash]

I agree with @reckoner165 on this. This enhancement also prevents accidental corruption of input files.

A possible solution is to have defaults for input and output directories but also make it configurable by having a way to provide input and output directories as environment variables (accessed via os.environ.get(...) or arguments (utilizing [argparse`](https://docs.python.org/3/library/argparse.html) - a utility for writing command line interfaces).