Modernize and fix long standing issues with package

[devvoh]

See https://github.com/devvoh/fixed-width/tree/dev-0.3.0-rewrite

This package was written well before I was comfortable with composer and packagist. Also, before I was heavily into testing.

Now, a more modern version of the package has been created, but I need input! Let me know what it's missing compared to the previous version (I implemented what I thought was necessary for my own hypothetical use cases) but by all means correct me!