Add mix task for #6

[dominicletz]

This pull request has three commits:

1. Adding a mix task so this can be used as mix duplex --threshold 7 and similar
2. Updates OptionParser usage and 'function_call' vs 'function_call()' to remove warnings with newer elixir versions
3. Largest commit applies mix format to the source tree to use elixir standard formatting

If you like we can split that up.

@junk2112