The Nyan Cat Wide Formatter

Hello @mattsears.

I've been using this awesome gem for some time, and I ended up missing something: I would like the cat to always reach the end of the terminal, thus replacing the fixed one-column-for-step for a variable value, proportional to the number of tests.

I have added a `NyanCatWideFormatter` which extends your classical `NyanCatFormatter` with this new behaviour.

Thanks for your time!