Reformat plugin 

A few minor changes.

1. Make `module.exports` an instance of the plugin so require('plugin')` can be used directly as a filter.
2. Remove extra closure around `date()` method.
3. Move `meridiemRegEx` to the global scope since it does not change.

This also wraps up the README changes in my previous pull request, so if you like both commits you can just merge this one.

Let me know what you think!