Transpile entire directories and create command line interface

[JSAbrahams]

Relevant issues

fixes #100

Summary

Add options for transpiling an entire directory of files instead of a single file.

• Uses input as either input file or input directory
• Uses output as output directory (error if not directory)
• If no input, defaults to using src in current directory (error if this doesn't exist)
• If no output, defaults to using `target in current directory (creates one if this doesn't exist)

Added Tests

Modified current tests to handle new command line interface.

[JSAbrahams]

It appears that the executable expects absolute paths as arguments instead of relative paths, which does not reflect the intended use-case. So this should be rectified.