Add argument to ordinal to support <sup> tags.

I got really sad that I couldn't display values in traditional formats on my blog, so I went searching for an answer and I found your lovely extension! But I noticed your ordinal filter doesn't add `<sup>` tags. So I thought I'd fix that for you :sparkling_heart: 

Here's an example use case:

    {% assign date_month = page.date | date:"%B " %}
    {% assign date_day = page.date | date:"%d" | ordinal:'sup' %}
    {% assign date_year = page.date | date:", %Y" %}
    {% assign date_abbr = date_month | append:date_day | append:date_year %}

This is what I use to render the dates on https://justinetunney.com/steve-jobs.html

Thanks in advance for merging. Also sorry about the newlines at the end of the file.