Allow customization of embedded tweets

[randycoulman]

Twitter provides several options for customizing embedded tweets. The full list is available in their developer documentation. Of particular interest are the ability to turn off embedded media and the ability to show only the tweet and not the surrounding conversation.

This PR adds the ability to provide optional keyword arguments after the tweet status URL.

The parsing code is modeled after some of the bulit-in tags in Liquid itself. I made one change from that pattern, though, because the attribute parsing code was also catching the URL, resulting in a data-https attribute being added.

I did not implement any kind of whitelist or blacklist for keys, though that might be desirable. I can add it in if you think it necessary.