Add alphabetical sorting of packages

This release adds a new feature that sorts entries in the requirements.txt file alphabetically as they are added.

Improvement to pip freeze check

This improves the search pattern on the output of pip freeze. In particular, pyp will now ignore case and consider - and _ equivalent when searching for matching package names.

Better error handling

This version adds better error handling and a helpful message about allowed commands.

First release

This is the first version of pyp.