Add link unfurling option

This is similar/the same as issue #17 but that has been dormant for a while. I'm creating this request as it's based off of latest master.

The changes are simple, basically just duplicating the code for LINK_NAMES and that's it!

As a note, the other PR mentioned not knowing what unfurling is. In this case it allows Slack to automatically grab information about a link sent to it and embeds it in the chat. For example, posting a .jpg would cause Slack to unfurl it by grabbing the image and embedding it into the chat.

The removes are simply adding spaces to make the code more readable as "unfurl_links" is longer than other options.